Hey Al,
Oberheim made a midi unit long ago called the "strummer". Iy would revoice keyboard chords to match a guitarists voicings and produce the strumming effect. Worked pretty good, too. They are sometimes on eBay for about a hundred bucks or less. As for riffs - you have to become "one" with the pitch bend lever and THINK like a guitarist. There is no substitute for "Learning" the proper technique for each intrument sound you try to emulate with a synth. MOST of why synths sound artificial is in the "performers hand" & NOT the programmer!
A good way to start is to set your pitch bender for a whole step, so you can do gentle bends of a quarter tone to a half, etc. Usually, bend UP to a note, and try to get it as smoooooooth as you can. Listen to your fav guitarists and copy licks 'till you get it. It's time well spent. You'll sound MUCH more realistic once you get the bender working right.Also, try to NOT play riffs that are IMPOSSIBLE on a fretboard. (Check a guitar chord chart if you need help)

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Uncle Dave