NiMh huh? hmmm . . . sounds like older gear. Still useable, but you're likely restrained by what it can do.
So what I hear you saying is the transmitter is not programmable? If so, all it has is the native mapping, and no mixing. A good chance you-can't-get-there-from-here with that gear. The problem with "all ports neutral" is that speed controllesrs have a different "neutral" -- they typically like neutral at 0% where other channels like it at 50%.
Does the package come with an external ESC or is it a brick transmitter (designed with the speed controller built into the receiver)? if it's a brick, then *at best* you could wire the motors together (assuming it wouldn't burn out the speed controller), but otherwise you'll need an ESC to run a second motor, matched to the motor.
Only other crazy option (if I understand what you're equipment is correctly and it's MODE 2), you could remove the centering springs from the rudder stick, get a ESC and plug it in (w/o power pin) to the rudder channel, then either twist the gimbal 45 degrees or hold it at 45 degrees. Full centered throttle becomes the top right, weak left thrust becomes up, strong right thrust becomes right, and off is bottom left.
If the TX is mode 1, you'll do the same w/ the aileron stick/channel instead of rudder, and plug the ailerons into the rudder, flying the tilted dual throttle on the right stick and aile/elv on the left.