All of these electronic kits work on the same principle - the stick or beater strikes a piezo crystal which produces a trigger pulse. The pulse triggers the electronic circuit in the 'brain' which makes the sound.
So using a different make of kick pad should be fine. My own kit is from the same stable as yours (though under a different brand name) and I have added several extra pads and cymbals from Yamaha, Roland and Alesis. They all work. To get extra sensitivity, I boosted a couple using a simple audio preamp (the trigger pulse is an audio signal) but the alternative is simply to hit the thing harder!
(How did I plug in those extra pads and cymbals? I bought a second-hand 'brain' from ebay and plugged the output of one into the Stereo Aux input of the other).