Yes....it is possible. Even before I started using interchangeable thumbs a few seasons ago, I had a mold of my thumb that was drilled into every ball I drilled, and just had my custom thumb put in an it-----the thing with that is that to do that your proshop guy will need to have "larger" sleeves ..the standard ones are too small to put a custom thumb in but if he doesn't have the larger ones in stock, they can be ordered that can accommodate a thumb mold.
Brad