some weird coder is going to tell you it's not possible. it is though. this belongs in modification discussion.
but I will help you
I have no idea to make a ball mountable by jumping upon it. however there is a work around.
look at a ball and do /getid. get the number it gives you. that number is x. then do the same while looking at the person you want mounted. to do yourself just orb. your ID will be y.
iirc the console command is this- x.mountobject("y, 1");
I'm on an iPod so I can't verify