I agree. I don't think this is possible.
What you'd almost need is a DMX merging system to inject both a console(with joystick control) into the stream coming from MyDMX, which is NOT a recommended thing to do with an asyncronous protocol.
You'd be better off with two sepparate DMX leads, one from MyDMX and another two your fixtures going to a console.
Honestly, what you want to do is more higher end console type stuff. Typcally, a light would not be taking out of a show, but could be quickly controlled while a show runs(or song or scene) and then hijack certain fixtures with a control like a joystick.
MyDMX is not that complex. It can do a lot, but not that.