You'd have to buy the ticket without an itinerary, I think. A ticket offices should have no trouble selling it to you.
The Conditions of Travel say "Your Ticket ... will be valid ... by any services ... over the shortest route which can be used by scheduled passenger services between the stations shown on your Ticket".
The probelm you're having is that retailers ignore the "which can be used by scheduled passenger services" bit, so don't accept that this is a valid itinerary.
I'm actually a bit wary of suggesting this because the routeing guide says to use some tables on
nr.co.uk to calculate the shortest distance, but the route you're using isn't listed there at all. I'm struggling to imagine a prosecutor arguing that a route is not the shortest one purely because they've left it out of their list of routes, though.
For completeness, the route can't be validated as a "mapped" route for Wolverhampton-Walsall because the only map combination for the journey (BI) requires travel via Birmingham, and because it entails a double back through Bescot Stadium and you would need an easement to allow that.