If the triac controller works the way I think it does, then a bridge rectiifer followed by a large smoothing capacitor to rectify the triac output to a DC voltage should work. (smoothing capacitor is probably not needed if the AC frequency is very high, but mains 60Hz and 120Hz is too low).
But from my understanding, a triac crudely chopping the AC waveform shouldn't be able to drive an AC motor. BUt you didn't say AC motor, you said AC/DC motor. It would be similar to a PWM signal used to drive DC motors, but instead of chopping a constant unipolar DC voltage, you are chopping the cycles of a bipolar sinusoidal voltage (which becomes unipolar when you rectify it).