Can I use a derailleur hanger in my old frame?
There are replacement hangers to adapt a modern deraileur to horizontal drops. I have never used one and can't say how well they work. BikeParts.com lists this Shimano compatable piece ID#: BPC109497 that  may do the trick. Talk to someone at the LBS (local bike shop) first,if you get it locally you will be able to try it for fit before buy it.