Can you break your toe/bone if you attempt to crack the knuckle of a very bad hammertoe?

Unlikely: Bone is pretty hard but what's more likely if you try this is rupture some ligaments or joint capsules. See a podiatrist.

Possibly.: It is possible, but more likely that you would strain/sprain the toe possibly damaging ligaments. If you have a "very bad hammertoe" then you should seek a consultation from a surgically trained podiatrist and have it properly evaluated. He/she can give you different treatment options.