before spending any money to buy pedals to emulate humbucker or fatten the sound(theres not much dedicated pedals doing that. Before anyone suggest eq, eq might fattern the sound, but it still doesnt make it sounds like humbucker imho...some of those which can do the single coil humbucker thingy, are part of multiefx/effect modelling device usually(roland vg series, line 6 variax or yamaha magicstomp),
without changing the characteristic of the clean sound l(by adding any distortion or fuzz), heres my suggestion
1) Install those humbucker pickup in single coil size, from seymour duncan if not wrong. How well, how good, how how. I dont know, either google round for the model or wait for some others in the forum here who use those to chip in. Those are humbucker, but in single coil size.
2) Cheapest way(as compare to buying more gear to get the fatter sound) and opening up tonal choices of strat with single coil to sound huge and fat, without changing the original look of the geetar itself. Rewire the whole thing to get any of the 3 single coil to be in series or all 3 single coil in series. Most geetar wiring is parallel. If you can or find someone to rewire the pickup in series and activated by a push pull pot, you will still have the 5 sound setting plus the single coil in series sound.
2 or 3 single coil in series will sound fattter and louder.
If you wanna more of fatter and choices in strat, go here
GuitarNutz 2 - Mike Richardson wiring with phase
i have tried the above and it works. If you wanna something less, just search around the forum there, there shud be some info on getting the single coil pickup in series wiring. If not, just google around.