You can put as many as 3 humbuckers in a Strat if you wish. You can choose two routes. One route is to get a new pickguard/scratchplate with cutouts for traditional double-wide buckers. The other route is to get the Strat style buckers which fit in the original openings without modification. The bodies of modern Strats are usually routed HSH under the scratchplate so the middle pup will pretty much have to be a single coil sized bucker. Some older Strats from the late 80's to the mid 90's had "swimming pool" routed bodies. With those you'd be able to do pretty much anything you can dream up. I've seen pix of custom made scratchplates on swiming pool guitars with 4 pickups mounted on them. Search the pup maker's web sites. You'll be able to find pretty much anything you want.

If you need a custom made scratchplate you can order those here:

http://www.pickguards.com/
Do you know anybody who does wiring

http://www.wdmusic.com/pickguards.html