I'm also curious, why the SME server community doesn't host a IRC chat on freenode, i've logged into the online chat section a couple of times and i've never seen anyone online there.
I'd rather have people discussing SME on Mailing lists and forums, which are searchable, IRC isn't. I wouldn't mind if we ax'd the chat applet in favour of IRC. If you or someone wants to set that up, please email staff@contribs to discuss.
There are some imap server to server migration scripts - I haven't used this, but looks like what you need -
http://www.linux-france.org/prj/imapsync/Please have a look at the old manual for SME6 - link on the documentation page (no SME7 manual yet) - and search the forums and mailing lists and I think you'll find most of the rest of the answers you are looking for.
I think you'll like SME7. It might be a bit of getting used to coming from a linux background - especially the fact that SME does everything most people need without having to use the command line. Be sure to check out all the options in the server-manager.