It's definitely possible but I'm not sure what the exact procedure is.
I suggest that you keep asking on the official phpBB forum and see if anyone can eventually help you either for free or for a small fee. As chances are, there's at least one person who has done that already and perhaps they could be able to help you out. I think MyBB has a tutorial on how to do that, although it probably won't help for phpBB but basically it's just making a PHP page that authenticates the information by connecting to the forum's database.
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