I've seen, and used, several different types of programmable transmitters and the first 4 channels in all of them were fixed. I would think, however, than an OpenTX transmitter might be programmed to put the throttle stick on any channel but that would be at the processor level, not at the user level. But the more important question is what channel your receiver's built-in ESC is on? With that answer you might also be able to use a mix to map your throttle to the ESC.