You may. :wink:
I'm using a retro-fitted Sigma 50mm 2.8 macro FD lens. Yes, FD. I bought it thinking it was the newer EOS version. Disappointed as I was, I decided to MAKE it fit my 20D. So I got an old FD extension tube, took the male connection off and screwed it onto a camera cap. I carved out the inside and now it works! It has a TERRIBLE flare problem, but usually macro doesn't have to deal with that anyway, and it's REALLY sharp.