Kinda, sorta.
I know you can assign a controller(typically a MIDI device) to channels. Like, let's say you have a joystick type controller on your keyboard(such as my old Korg DS-8), you can assign your X and Y channels to MyDMX. Short of that, a Korg Nanopad has an X/Y pad and can do the same thing, but it's a bit dodgy assigning either way due to the non-exact nature of people.
As far as "on the fly", again, yes and no. You need to find what works best for you, either HTP or LTP. As scenes come and go, things may or may not changed for you automatically.
What you can do is just make more complex scenes. MyDMX is more "snapshot" based, so even in a complex scene with many steps, it's basically moving from step to step as if running a series of "frames" in a movie. It is what it is, it does what it does.
The problem comes in with "well, I want to do this, and then I want to bring this other element in and out". That's a but more typical of what you want to accomplish here, where you are trying to run more than one scene concurrently. Unfortunately, MyDMX does not support that.
But, if all you want to do is tweak on the fly, then I suppose using HTP and LTP might be your options, coupled with a MIDI controller.
Another option might be a DMX controller and a DMX merger combined with MyDMX to group additional signals together.