Re: 7 string -Adding another single coil to a dual humbucker guitar
Where to start?  Start with either taking your body to someone to be routed or see about buying a routing template for it.  Places like stewart mac sell them.  After you get it routed drill an appropriate channel for the wires.  Then add a new switch and you are off.
Most companies now make 7 string singles so that isnt much of an issue.