Some types of takedown connection are only suitable for installation when the bow is being constructed, but others (such as the hinge connection on Ray's bow) can be added later if the riser style is appropriate for its use. I have some misgivings as to whether your bow has a riser that would be conducive to a hinge or a sleeve connection, which are the ones most easily added to existing bows. If I were in your situation, I would be looking for a used takedown bow in your price range. Obviously, the classified section would be a good place to start, and a nice 3 piece bow would probably be the easiest to find.
Allan