[Xiaomi Mi 5s ROMs, Kernels, Recoveries, & Other De] KopiJahe:
That's what I did for my zips:

1. Open the zip with 7-zip or whatever program you use (winrar, winzip, etc.)

2. Delete unnecessary parts: "cust" folder, boot.img, file_contexts, system.new.dat, system.patch.dat, system.transfer.list

3. Modify META-INF/com/google/android/updater-script accordingly

For example:

Original updater-script:

getprop("ro.product.device") == "capricorn" || abort("E3004: This package is for \"capricorn\" devices; this is a \"" + getprop("ro.product.device") + "\".");
ui_print("Target: Xiaomi/capricorn/capricorn:7.0/NRD90M/7.6.6:user/release-keys");
show_progress(0.100000, 5);
mount("ext4", "EMMC", "/dev/block/bootdevice/by-name/cust", "/cust", "max_batch_time=0,commit=1,data=ordered,barrier=1,errors=panic,nodelalloc");
package_extract_dir("cust", "/cust") || abort("Failed to extract dir from \"cust\" to \"/cust\".");
set_metadata_recursive("/cust", "uid", 0, "gid", 0, "dmode", 0755, "fmode", 0644, "capabilities", 0x0, "selabel", "u:object_r:system_file:s0");
show_progress(0.200000, 10);
package_extract_file("boot.img", "/dev/block/bootdevice/by-name/boot");

# ---- radio update tasks ----

ui_print("Patching firmware images...");
package_extract_file("firmware-update/cmnlib64.mbn", "/dev/block/bootdevice/by-name/cmnlib64");
package_extract_file("firmware-update/cmnlib.mbn", "/dev/block/bootdevice/by-name/cmnlib");
package_extract_file("firmware-update/hyp.mbn", "/dev/block/bootdevice/by-name/hyp");
package_extract_file("firmware-update/pmic.elf", "/dev/block/bootdevice/by-name/pmic");
package_extract_file("firmware-update/tz.mbn", "/dev/block/bootdevice/by-name/tz");
package_extract_file("firmware-update/emmc_appsboot.mbn", "/dev/block/bootdevice/by-name/aboot");
package_extract_file("firmware-update/lksecapp.mbn", "/dev/block/bootdevice/by-name/lksecapp");
package_extract_file("firmware-update/devcfg.mbn", "/dev/block/bootdevice/by-name/devcfg");
package_extract_file("firmware-update/keymaster.mbn", "/dev/block/bootdevice/by-name/keymaster");
package_extract_file("firmware-update/xbl.elf", "/dev/block/bootdevice/by-name/xbl");
package_extract_file("firmware-update/rpm.mbn", "/dev/block/bootdevice/by-name/rpm");
package_extract_file("firmware-update/cmnlib64.mbn", "/dev/block/bootdevice/by-name/cmnlib64bak");
package_extract_file("firmware-update/cmnlib.mbn", "/dev/block/bootdevice/by-name/cmnlibbak");
package_extract_file("firmware-update/hyp.mbn", "/dev/block/bootdevice/by-name/hypbak");
package_extract_file("firmware-update/pmic.elf", "/dev/block/bootdevice/by-name/pmicbak");
package_extract_file("firmware-update/tz.mbn", "/dev/block/bootdevice/by-name/tzbak");
package_extract_file("firmware-update/emmc_appsboot.mbn", "/dev/block/bootdevice/by-name/abootbak");
package_extract_file("firmware-update/lksecapp.mbn", "/dev/block/bootdevice/by-name/lksecappbak");
package_extract_file("firmware-update/devcfg.mbn", "/dev/block/bootdevice/by-name/devcfgbak");
package_extract_file("firmware-update/keymaster.mbn", "/dev/block/bootdevice/by-name/keymasterbak");
package_extract_file("firmware-update/xbl.elf", "/dev/block/bootdevice/by-name/xblbak");
package_extract_file("firmware-update/rpm.mbn", "/dev/block/bootdevice/by-name/rpmbak");
package_extract_file("firmware-update/splash.img", "/dev/block/bootdevice/by-name/splash");
package_extract_file("firmware-update/NON-HLOS.bin", "/dev/block/bootdevice/by-name/modem");
package_extract_file("firmware-update/logo.img", "/dev/block/bootdevice/by-name/logo");
package_extract_file("firmware-update/adspso.bin", "/dev/block/bootdevice/by-name/dsp");
package_extract_file("firmware-update/BTFM.bin", "/dev/block/bootdevice/by-name/bluetooth");
package_extract_file("META-INF/com/miui/miui_update", "/cache/miui_update");
set_metadata("/cache/miui_update", "uid", 0, "gid", 0, "mode", 0555, "capabilities", 0x0);
run_program("/cache/miui_update");
delete("/cache/miui_update");
show_progress(0.600000, 250);
ui_print("Patching system image unconditionally...");
block_image_update("/dev/block/bootdevice/by-name/system", package_extract_file("system.transfer.list"), "system.new.dat", "system.patch.dat") ||
abort("E1001: Failed to update system image.");
show_progress(0.100000, 2);
unmount("/cust");
set_progress(1.000000);

Modified updater-script:

getprop("ro.product.device") == "capricorn" || abort("E3004: This package is for \"capricorn\" devices; this is a \"" + getprop("ro.product.device") + "\".");
ui_print("Target: Xiaomi/capricorn/capricorn:7.0/NRD90M/7.6.6:user/release-keys");
show_progress(0.100000, 5);

# ---- radio update tasks ----

ui_print("Patching firmware images...");
package_extract_file("firmware-update/cmnlib64.mbn", "/dev/block/bootdevice/by-name/cmnlib64");
package_extract_file("firmware-update/cmnlib.mbn", "/dev/block/bootdevice/by-name/cmnlib");
package_extract_file("firmware-update/hyp.mbn", "/dev/block/bootdevice/by-name/hyp");
package_extract_file("firmware-update/pmic.elf", "/dev/block/bootdevice/by-name/pmic");
package_extract_file("firmware-update/tz.mbn", "/dev/block/bootdevice/by-name/tz");
package_extract_file("firmware-update/emmc_appsboot.mbn", "/dev/block/bootdevice/by-name/aboot");
package_extract_file("firmware-update/lksecapp.mbn", "/dev/block/bootdevice/by-name/lksecapp");
package_extract_file("firmware-update/devcfg.mbn", "/dev/block/bootdevice/by-name/devcfg");
package_extract_file("firmware-update/keymaster.mbn", "/dev/block/bootdevice/by-name/keymaster");
package_extract_file("firmware-update/xbl.elf", "/dev/block/bootdevice/by-name/xbl");
package_extract_file("firmware-update/rpm.mbn", "/dev/block/bootdevice/by-name/rpm");
package_extract_file("firmware-update/cmnlib64.mbn", "/dev/block/bootdevice/by-name/cmnlib64bak");
package_extract_file("firmware-update/cmnlib.mbn", "/dev/block/bootdevice/by-name/cmnlibbak");
package_extract_file("firmware-update/hyp.mbn", "/dev/block/bootdevice/by-name/hypbak");
package_extract_file("firmware-update/pmic.elf", "/dev/block/bootdevice/by-name/pmicbak");
package_extract_file("firmware-update/tz.mbn", "/dev/block/bootdevice/by-name/tzbak");
package_extract_file("firmware-update/emmc_appsboot.mbn", "/dev/block/bootdevice/by-name/abootbak");
package_extract_file("firmware-update/lksecapp.mbn", "/dev/block/bootdevice/by-name/lksecappbak");
package_extract_file("firmware-update/devcfg.mbn", "/dev/block/bootdevice/by-name/devcfgbak");
package_extract_file("firmware-update/keymaster.mbn", "/dev/block/bootdevice/by-name/keymasterbak");
package_extract_file("firmware-update/xbl.elf", "/dev/block/bootdevice/by-name/xblbak");
package_extract_file("firmware-update/rpm.mbn", "/dev/block/bootdevice/by-name/rpmbak");
package_extract_file("firmware-update/splash.img", "/dev/block/bootdevice/by-name/splash");
package_extract_file("firmware-update/NON-HLOS.bin", "/dev/block/bootdevice/by-name/modem");
package_extract_file("firmware-update/logo.img", "/dev/block/bootdevice/by-name/logo");
package_extract_file("firmware-update/adspso.bin", "/dev/block/bootdevice/by-name/dsp");
package_extract_file("firmware-update/BTFM.bin", "/dev/block/bootdevice/by-name/bluetooth");
package_extract_file("META-INF/com/miui/miui_update", "/cache/miui_update");
set_metadata("/cache/miui_update", "uid", 0, "gid", 0, "mode", 0555, "capabilities", 0x0);
run_program("/cache/miui_update");
delete("/cache/miui_update");
set_progress(1.000000);