Yes! It is relatively simple to turn your stable Rockbox install into a development one. To do that, you need to first uninstall the bootloader for your device by going through the utility's "Backup & Uninstallation" tab. In there, you then need to click the "Remove Bootloader" button.
Once done, you can go back to the utility's "Installation" tab and pick the "Development Branch" in the version drop-down menu. From there, you then need to select and install both the bootloader and firmware components for your device.