Phpnuke already comes with a phpbb forum intregrated into. I assume you don't want to use that and you would rather use invision powerboard?
Well, install both portals to the same MySQL database. Then find the tables that contain each portals user information. Compare them and find the core common fields that both contain. Go through invision's code and see if you can edit it so that it looks at the user table for phpnuke. You will probably have to adjust the table structure and some code in invision in order to get it working properly. That is one way I know you could do it. That does however assume that you can successfully edit the invision code, which I have never tried.
Also, I think there might be some sort of mirroring command within MySQL that will mirror certain fields between tables, but I am not sure what it is.