if I'm correct, you want an extra input field which gives an extra explination to your subject?
some things that you'll need then:
-new mysql field inside phpbb_posts
-editing in the posting of messages (editing basic functions)
-editing the loading
that's all but it might take some time to make it.
If nobody starts on this, I'll do it but that's going to take time since I don't have much spare time...