Question about using (and maybe extending?) a prusa mendel i3 for prosthetics.

I’m interested in learning about 3D printing, and hopefully one day participating in the e-nable projects or something similar. I recently found a decent deal (I hope) on a used prusa mendel i3. Whether I go with that one or buy a new kit, my question is the same; can I put a substantially longer leadscrew and set of rods on the bed to increase its build volume (for such things as forearms)? Would I have to slow it down, or print and install mounts for thicker rods? What’s the practical limit (is there a chance I could one day print all the parts for an arm in one shot)? http://ift.tt/2bjlMK0