Um,
I don't know if there's an easy way to do that really...I'd recommend adding a new switch entirely, as in the 'neck on' mod:
http://www.guitarnuts.com/wiring/allpickups.php
With that you can switch on the neck PU with the bridge.
I did roughly the same by using a 3-position switch for neck and bridge and wired the middle pup to it's own volume pot. That way I could get any combination with just a 3-pos sw and the 2nd volume.
Dee
"When life's a biatch, be a horny dog"
Amps: Marshall JVM 410H w/ Plexi Cap mod, Choke Mod & Negative Feedback Removal mod, 4x12", Behringer GMX110, Amplitube 3/StealthPedal
Half a dozen custom built/bastardized guitars all with EMG's, mostly 85's, Ibanez Artwood acoustic & Yamaha SGR bass, Epiphone Prophecy SG, Vox Wah, Pitchblack tuner plus assorted pedals, rack gear etc. for home studio use.