You need to wite a conversion tool in php.
The tool should take all the user stuff and copy it to another database i n the format of phpnuke/wordpress.
I am not sure if it is completely possible
it's easy and it's free, login to your wordpress blog
then click on "Import" choose RSS, phpnuke has rss backend download that as .xml file and then import as a file from your hard disk.
I don't think you can completely transfer a phpnuke site to wordpress.
They are 2 different scripts with different goals. phpnuke is a portal and wordpress is a blogging script.