If you extract the files from a .hak pack and place those extracted files in Neverwinter Night’s override directory, you can use that hakpack in any of the standard BioWare campaigns (with the usual disclaimer that hakpacks may break stuff, you will actually need to put in new merchants to use any of the new items, etc.) This is very useful for non-intrusive hak-pack changes, like the jiggly hi-poly human torsos models.

You can use tools like NWN Explorer or NWN Viewer to extract the files from a .hak pack.