That's right, the Admin Move Mailbox tool can only move mailboxes between servers in the same site in 5.5.
Really, the only way to move a mailbox between sites in 5.5 is to delete and recreate it. This causes all sorts of issues, because the site name is part of the underlying Object Distinguished Name of the mailbox, so it breaks replies to old mail unless you also add a X.500 secondary proxy address to the new mailbox. Also you have to DirRep between the sites after the delete and before the create if you want to be able to give the new mailbox the same SMTP address(es) as the old mailbox. And then you've got the contents of the mailbox to consider, Exmerge is a very good tool for exporting the extire contents of a mailbox to PST and back again into the new mailbox - I use it even for just one mailbox, it beats messing around dragging folders in a client. Bear in mind though that it breaks single instance storage, probably not a big issue for just a handful of mailboxes.