I saw some hiking forums that mentioned Nikwax for making other (non-DWR) garments a bit water repellent, but it won't make them as good as something designed and made with an actual DWR fabric.

I washed my g/f's nylon windbreaker with my GTX hiking raingear when I was waterproofing stuff last fall, and she's had no problems with it waterlogging, which it used to do quickly. I suppose it would depend a bit on the weave of the fabric, too. I doubt you could Nikwax wash a cotton t-shirt and stay dry with it.