Yes, you can use php nuke as a portal for UBB.classic. But the question is, do you have the knowledge and time to do it. It would require massive modding of the UBB or php nuke in order to integrate the two. You would either have to modify the php nuke code so it uses a flat file system based off of your member files from UBB, or modify your UBB to use the php nuke members table from the database. Doing either of these is probably not worth it as it would take rediculous amounts of time. Like Sub Zero said, if you know a good amount about UBB, you could develop your own portal which would be much easier than modding the member systems of one of the scripts.
I would recommend you just stick with the content islands, or else try out the ubb_portal.pl.
