IIRC, my idea about the crystals was that they:
1. regenerated antimatter (and matter fuel) when m/am energy is applied to them by possibly borrowing from an antimatter universe
2. they store a charge like a giant capacitor or battery
I think fusion power could charge one up but it might not be energetic or special enough to create antimatter in return, so a fusion-powered ship with crystals would get the buffered power benefit but would still have to gather up fuel on it's own, IMHO
@Bob - I think there is ample evidence in TOS and in TWOK that points to the crystals being separate from the whole m/am reaction.