If your email account you're trying to repopulate on the BB is an IMAP account there is a trick you can try. I've used it with Apple's MobileMe email account on BIS and it has definitely worked.
On your computer, move the old messages you want to transfer to another folder. You can create a new folder called "saved" or something like that, just for the procedure. When the messages are all there, mark them as unread. Then drag them back to your inbox. You might have to wait, but sometimes (depending on your email provider) this will trick RIM's servers into thinking they're new messages and they will download again to your BlackBerry. edit: I just confirmed this with my MobileMe account.