Pretty easy. The pickups go to the selector switch first, so you've got a common output. So if you make the volume pot a switched pot like the tone pot, then you just take the selector switch output to the common of one side of the switch, take the 'down' position switch output pole back to the normal volume pot connection, and the 'up' switch position output pole direct to the jack socket tip connection.
But you also need to break the connection on the output side of the volume control, otherwise you've got a path back for the output signal through the volume pot to the tone control. Remember that when the volume pot is at 10, the pot's input and output tabs are directly connected, so the tone control would still be in circuit.
So, when the switch is pulled up, the pickup output bypasses the volume control and the tone control connection,. When down, it's business as usual. So the blower switch operates for all pickup selections.