You can trim off the neck of a figure pretty easily just get it hot (boiling water) and then cut the neck off. Use a ball joint head as a referance and cut under the hair line and around the back of the chin. Then you can just drill up inside the head to make it fit on the long neck and use some apoxie or whatnot to fit the ball joint.
I've done it. You have to dremel out a hole in the head, then once you know it'll fit, build up a little green stuff (putty) around where the end of the ball joint "peg" in the neck is so it'll snap in. It's a pain, but if the results work, definitely worth it.
the best thing to do would b to drill a large enough hole in the head to insert the little rubber area found in the HT neck adapters by actually cutting that out and glueing it in your neckless head so it will fit ok