Looking at my web stats, I saw a few visitors from Google who were looking for information on how to get old messages onto a Blackberry. Since I didn't actually have that information, I'll add it here.
Suppose you have some old messages in your IMAP inbox which have been purged from your Blackberry because they are too old, and you would like to get them back so you can reply while on the subway. Just copy them from your inbox back to your inbox. You can do this in a single action with mutt--using save will copy the messages and delete the originals. But Thunderbird doesn't seem to allow you to create a copy of messages within a folder so you will have to move (copy and delete original) the messages to another folder, and then copy them back to your inbox.
The copies will then look like new messages to the Blackberry service, and your Blackberry will download them. If, within your non-mobile MUA, your sort your inbox by date or date-then-thread (mutt's threaded mode), your inbox should appear the same as before making the copies, but if you sort your messages in the order they exist in the IMAP store (including Thunderbird's threaded mode), the copies will appear at the end (or beginning, depending on your sorting direction) of your inbox.
The state is that great fiction by which everyone tries to live at the expense of everyone else. - Frederic Bastiat