I've been curious as well. More specifically, I was curious about adding kevlar patches and armor pockets to existing street jackets and pants.
I've started evaluating what pieces of clothing that could easily be converted, and for the most part, very few jeans are constructed of 8 oz. denim or heavier. Only the nicer brands have double stitched outer seams as well (try pulling the legs apart and you'll easily see that most jeans just have single stitching on the outseam).
I think that jackets are going to be more difficult, but having been a mountain sport enthusiast, I have several ballistic nylon jackets that could be good candidates (double stitched seams, Schoeller fabric, ballistic nylon elbows and shoulders).
Sadly, I am rather unskilled with a sewing machine, so it will be a project for when the kids are off to college and I'm no longer swamped with other projects.