Which Passat? The B5s already came in AWD versions, so not really worth the effort. But yes, it *can* be done. Just a lot of work, since the rear pan and subframe are completely different.
I am working on converting my 4mo B5 wagon from the 1.8L gas engine to the 2.0L diesel, but need to get the transmission stuff sorted. It is looking like it will be converted to a manual even though its existing slushbox works fine.