Unfortunately it doesn't look possible to me starting with the wiring in that diagram.
In that wiring scheme, if you added a pot/resistor for either the "bridge only" or "neck only" switch position, it would screw up the other two switch positions.
You'd have to rewire the switch like the other diagram, but if you have some jumpers, you might want to add a pot to test it first to see if it's worth your trouble - but things would only work correctly in one switch position.