there is a tool for migration from Imail to mailEnable. Look under downloads -> Tools and Utilities
but if you have standard mBox format, where all mails are stored in one text file for each account, huh, might be a problem.
I had that situation.
There was no tool for migration, so I installed TheBat! mail client on server, and manually typed-in all acounts data, usernames and passwords. then I created a RULE in TheBat!, which redirected all received mail to receipents address. But since server first looks locally, all mails were delivered to local accounts, not back to where they were read from.
The advantage of such method is that you do not need to worry some mails shall be lost during DNS changes - you then just simply click send&receive and all mail, which accidentally ends on old account is automatically migrated to new account on your server.