You will then be able to create sub-folders under that folder. The IMAP server can read your MH folders directly. To do this, add a second mail server configuration to your client. When you open that server, it should show all your MH folders. When you open an MH folder, the IMAP server reads every message in the folder.
Big folders can take a long time to open.
The IMAP server uses the first message in each IMAP folder to store IMAP folder settings. Normally, the IMAP server doesn't show you that message. You can delete the message. The IMAP server will re-generate it as needed. But, you should disable all POP access to your mail.