Ok. Here's the deal. With your DTX 500 module, you can add up too 4 additional single zone pads.
On my kit, I've added an extra mounted tom and an extra floor tom, so my set is equivalent to a 7 piece kit. I've also added an additional cymbal, which is single zone (but hey, when I hit a crash cymbal, I really only use it for its crashing abilities anyway). The additional cymbal also will not be chokable.
Also, for myself, I had an issue with my snare pad top layer getting some small bubbles in it. Without missing a beat, Yamaha sent me a brand new 12" snare pad and told me not to worry about sending the old one back. So, I've removed the 3 zone capability from my other cymbal and use it on my other snare pad, effectively turning it into a 3 zone effects pad.
So, I've got a 7 piece kit, with 3 cymbals and 1 3 zone effects pad.
BTW, if you just want to expand to one more cymbal, you can plug it into the back of the bass drum output. But if you want to add more pads later, you'll have to buy the "stereo plug that comes out to a left and a right mono"
Peace, Defender