A good strategy is to create all of the user accounts on your new Citadel system and then copy from the old system to the new system using IMAP. Unless you're just moving a tiny amount of data, though, you probably don't want the hassle of doing this manually using your desktop email program.
We recommend Gilles Lamiral's IMAPsync program, which works well with Citadel. Installation of this program requires some amount of comfort with the unix command line (such as installing Perl modules).
We also recommend Armin Diehl's 'IMAPcopy' program for this task. It is not as reliable as IMAPsync, but it is much easier to install and configure.
There are also the Imap Tools or OfflineImap, which parse several on-disk formats such as unix mbox or Mozilla Thunderbird. These programs have not been tested by the Citadel developers but you may find them useful.