Well actually you don't NEED to be able to palm the ball to dunk as you can dunk with 2 hands and squeeze the ball while jumping. It does require a lot less vertical leap though of course and the best way to get better is just practicing like many things. Get a hold of a size 6 ball to begin with and palm that regularly and hold it, when it gets easy go up to size 7 and practice that. Practice is the key to learning things, my hands are only 19cm long and I can palm a size 7 basketball in each hand at the same time now and hold it as well as palming the ball of the bounce. I began with hardly any grip and unable to palm the ball at all.