Except that's the opposite of what Spot wants...
Extracting the numbers from the string isn't a problem, but AFAIK there isn't any way to reference a variable via a string, so you might have to do it by hand:
select var$
case "p\x": p\x=p\x+value
case "p\y": p\y=p\y+value
case "p\z": p\z=p\z+value