I have to agree with Frank from ProSounds above: realistic strumming does require subtle and precise MIDI events. But I'd like to offer an alternative to using a static .mid file to get there. Controlled arpeggiators with a *lot* of variations can get you there as well. If you only *set and forget* an arpeggiator, it's going to sound mechanical. But controlling the shapes, on/off, length/rate, and other parameters over MIDI can get you very close with practice. The best sources to use are those beneath your fingertips as you play your keyboard controller.
Those might include velocity, release velocity, keyboard mapping, and aftertouch. They keep you concentrated on the phrasing, without having to reach all over for a particular control. I did a live guitar piece in Rapture that I'm going to point to [
(BAGEED) here]. While the intent was to take realistic acoustic guitar and bring it into synth territory, there's enough that's 'realistic' there to demonstate what I'm talking about.
Project5 has a decent set of per-track arpeggiators with provision for external control. I've also heard good things about Sonar's new MIDI FX arp. These won't generate editable MIDI without a hack, but they can still be useful in emulating guitar-specific techniques. In the song linked above, I used a single KB controller with some onboard (hardware) arpeggiators to pull off a live rendering.
I'll also point you to a tutorial that I just put up here that describes the patch setup that I used. You don't have to go this far in emulating a guitar, but it does demonstate how to approach a natural instrument reproduction. I hope there's a few tips in there that you find to be useful.