I think I have a handle on your answer, but it requres some glossary terms first.
The Zendrum sends all kind of MIDI information, but the kind of information it sends that we hear as sounds is the MIDI Note Number. Note Number 38 is commonly a snare drum, 36 a kick, etc.
The FC-300 does not look like it transmits Note Numbers; that's why you're not hearing any sounds when it's put between the Zendrum and the module.
If you want to change kits in the Zendrum through the FC-300, you can just connect the FC to the Zendrum's MIDI IN. If you want to change kits in the module, you may need to get a 2-in/1-out MIDI box. I'd recommend the MIDI Solutions Merger. I don't have one but I've heard good things of the company:http://midisolutions.com/prodmrg.htm
There may be a way to make the control chain go FC-->ZX-->TD-6V, but without having all the units available to me, I can't give you much help with that. Hope this was of some benefit.