So this actually works in a roundabout way. Well, at least I was able to get it to work in my case. So in my example, I have a Novation LaunchKey 49 MK1 and I was trying to get the main volume slider to map to the main volume in S1. So I got to the point where I learned the control and set it up as a slider, but I couldn't find a way to assign it - the "Assign Command" option was missing... Well, I found that if I actually right-click on the main volume slider itself in S1, there was an option to assign that slider to the control that I just mapped! So that worked for me, hope this helps. Good luck!