DCL is fairly rudimentary
If you're not able to construct your pop up list without a slider using the typical DCL controls, then I doubt you're going to be able to hardcode a way to do this into your DCL. It's just old, and not updated, and has a lot of problems that have required many persons to go to other GUI methods. That said, you could simply divide your list into two portions, preferably with some sort of distinguishing factor, i.e. a size and b size, (although that's arbitrary, not sure the contents of your list at this time) and create two popup list boxes, would probably reduce the contents of each list to where they can be shown without a slider.
Additionally, I would be careful when doing this, because monitor size may come into play here. May want to test your DCL and lisp on a machine using a regular size monitor, a "large" monitor as well as a small monitor, to see if there are any changes in the way the slider appears between the 3 setups. If it appears differently, then you may want to reconsider your methods because it would be a shame to spend so much time on having these appear correctly on your machine just to find out that your machine is the ONLY machine that will have them display correctly. Unless, of course, that's what you're after anyways