With Orange Vocoder you can use an external carrier by sending both the voice and the carrier to the vocoder, what you do is a full pan to the left with the voice and full pan to the right with the carrier signal. Then within Orange Vocoder you dissable the internal synth (theres an option for it on the right top side of the window)
I know other vocoders works like this too, like the Mda Vocoder.
Ive never tried vocoding in Madtracker, but Im sure it works just fine.