There is indeed. There are 2 staff car parks, a small one beside platform 6 that at least when I was there was only for the drivers and TM's, and a larger one hidden away at the back of the surface car park beside the multi-storey (it's actually accessed from a completely different road to the station) for everyone else. When you're passing through Wolves, ask the station staff for a form and you'll get a card that opens the barrier.