I do not know of a convertor for PostNuke to phpBB3 so you will have to go to phpBB2 then phpBB3. Since PostNuke is quite similar to phpBB2, it shouldn't be difficult to modify the phpBB2 to phpBB3 convertor to do the conversion.
I have seen where people have had problems with passwords and private messages, but I have also seen where there has been success with these. The problem with private messages probably is that the tables were not copied over to phpBB2. I don't know about the password problem. It seems that PostNuke uses MD5 encryption which is what phpBB2 uses, so it should not be a problem when converting to phpBB3. If PostNuke does use a different encryption method, phpBB3 can be modified to accept the encryption method that PostNuke uses.
This topic shows a successful PostNuke to phpBB2 conversion, PNphpbb to phpBB
There is also this Knowledge Base article, DePostNuking The Database