If you want to Upload MBOX to IMAP Server then you should try SysInspire MBOX Converter Software. It has the ability to convert MBOX files into a different file format and after the conversion, you can easily upload MBOX files into IMAP Server. It has many features that help to make conversions of NSF files and also provides a free demo version of the software in which every user can convert their 25 items from each MBOX file without any complications.

More info:- https://www.sysinspire.com/mboxconverter/