You can import other mailboxes but that is really not syncing. You get all these other mailboxes.
I don't use IMAP but as z says that is what it is made for. You can do pretty much the same thing with POP accounts as long as you set all the computers up to NOT remove from the server. The benefit to me with POP is you can set it to delete under certain conditions like if removed from my inbox, so if I delete an email it will not show up to be deleted on another computer when I launch mail.
Both of these solutions are going forward so they not be want you are looking for.