if ultimatebb stores it's data in mySQL form, it's certainly possible.
Not to be an ass, but Gizmo, you're completely wrong when you say that two programs written in different languages or scripts are incompatible.
It's the form in which the data is stored; if PHP can read it, it's compatible. There's very little cross-script interaction in a given instance of a script being called.
xero: find out how ulatimatebb stores it's data, find out what PHPNuke needs, write a script to convert the data from one to the other, and replace all calls to that data with calls to the functions you defined in the new file.