I recall I did this using the Import function in Seamonkey (a cousin of Thunderbird) and it worked. The mbox format is the same, so you can use Thunderbird after, although I like Seamonkey better.

Also, you can create an imap account and synch the pst to that, then access everything with Thunderbird. If you want mbox, you'd have to create another account with Tbird and copy everything over to that.