In that case you've got a couple files with the .ABD extension (.EBD when you're using an encrypted message base)
Close TB
Copy those files to your current mail directory
Start TB
Open your address book.
When not all of your old addresses are included, pick in the address book menu:
File -> Open address book -> Pick your old AB.

This won't work when your old message base was encrypted and your new message base is unencrypted or encrypted with a different password.