Oh, I think I'm seeing what you are after now. A P-Rails won't actually sound like a traditional Strat pickup, though. Might be a cool sound, but an entirely different animal.
You COULD integrate some extra resistance in series or some capacitance in parallel to make it warmer sounding, but only in the "bridge alone" position, leaving the "bridge/middle" position unchanged, if you use one of the "Super Switches" that has the extra wafer.
https://www.stewmac.com/How-To/Onli...nd_Electronics_and_Wiring/Super_SwitchI1.html Or just find a bridge pickup that's a fatter and warmer sound. It'll still sound thin and quacky when combined with the middle in a Strat. That's just the nature of Strat pickups/wiring.
Of course, there's always the option to just have a tone control wired up to the bridge pickup and roll that tone back when you are on the "bridge alone" position. That's a free mod.