Castor oil wont add more bubbles to MP. In fact, it may dampen bubbles because its an oil.
Realistically the only thing that will probably work is adding a synthetic detergent to it. But, then by FDA labeling laws it will not be considered soap any more and will have to be labeled as a cosmetic.
When I make MP I add a tspn ppo of Dr. Bronner's liquid soap to it to add some bubbles, but I don't sell, so don't have to worry about legal/labeling issues.
Oops, sorry, that was meant to be 1 tspn of Dr. Bronner per lb of base. You could probably increase it a little bit if you are not adding other stuff. It does seem to make things a bit more bubbly, worth trying ...