Normally you can buy a ticket from any ticket office from the TFL Boundary to Walton (NOTE : Not all machines offer this). However Surbiton is actually the boundary, so Surbiton to Walton is the same price as TFL Boundary to Walton. You can purchase a ticket between any two stastion locations from all ticket offices, and most ticket machines.
If you purchase a ticket from the Boundary to Walton then the train doesn’t have to stop at Surbiton, however if you purchase a ticket from Surbiton to Walton then the train must stop at Surbiton.
However virtually all Walton trains stop at Surbiton, so that shouldn’t be an issue.