If you are playing live, it is very difficult to do. If you are recording, there is software out there. Rhythm 'N Chords which interfaces with Cakewalk has a strum library and lets you create strums by using the left hand to make the chord and pressing down on different keys with the right hand for different strum types (up, down, bass, etc.) These strum chords resemble guitar chords, and not keyboard chords.

It's very tough to imitate a guitar strum live, but strides can be made, for sure.

Beakybird