Just make sure that the table prefixes start with "phpbb_" instead of whatever the phpNuke default would be.
It also may depend on how the integration works. If the code has been modified to use the phpNuke users table(s) entirely, then you might not be able to export users.
Also note that you'll probably want to use the 6.5.1 beta (or wait for the final release) for your importing, as there are a handful of bugs in the 6.5 importer that can cause problems.
