Putting anything inside a ThunderB reduces the amount of room for the CO2 to expand and therefore reduces the amount of time it take for it to detonate. Meaning, it could go off well before it gets to the intended target or could even go off in your hand (I've seen this happen).
That aside, ThunderB's tend to zip open on one side and/or around part of the top. You have no idea where it will open though. It could very well blow it's BB's into the ground, do absolutely nothing. We've tried them with talcum powder and the colored talc S-Thunder sell and all it really seems to do is mark where the grenade went off and give you a starting point of where to look for it. Anything more than two teaspoons is a waste. We've never really noticed any appreciable "cloud" of talc when it went off, just a little puff. Adding more didn't do anything.
ASC Age Verifier for Red Deer & Area Alberta