You might need to experiment some, but I think you could just soak some percentage of the charcoal in water, or pour a measured amount of water in the hopper with the charcoal. You could also quench the charcoal with water, and store it slightly damp. The WK and most wood gasifiers are designed to condense excess water anyways, and the steam will distribute itself evenly as the charcoal heats up. Doing this, I think you could get by with no steam injection system or other modifications.
If the hopper exceeds 250 F, you’ll need to add more water. You will also notice a drop in power if the water runs out, due to the lack of hydrogen.
Bigger pieces and as “undercooked” as possible will be better. What kind of charcoal would it be?
And, just curious, why are you trying to do this?