I usually seal maple with shellac, then oil. Walnut, I have not sealed with shellac, but I don't see why not. Rub it on like French polish(look up YouTube), across the grain, and the shellac would fill the pores.
I've done Lin-Speed on a more modern walnut stock, but that was many coats, with wet oil sanding in between to slowly fill the grain while removing the surface build-up of finish. Laborious, but beautiful. Impossible to finish carving in this way.