Yeah, I agree with art. Nearly anything is possible with a VEX kit...or two.
It would be easier to build a 4 or 6 legged robot than a biped. With a biped, it might fall over due to being unbalanced and a gyroscope or two might have to be used to keep it balanced. You could use programming now with the VEX kits so it shouldn't be too hard though. You may not even need a gyroscope if you get all the timing right for the different movements, like the bending of the knee joint, the hip movement, and the ankle rolling to make it walk.
Here are some bipeds if you're interested in different designs.
Good luck making one. Should be somewhat of a challenge.