Is it legal to use the cage in MMA fighting?

no, you just cant grab it. like if your up against it and someone is about to pick you up and slam you, you cant hold on to it so you wont get slamed. God bless

In the MMA is it legal to use the cage as a weapon?...such as throwing ur opponent into the walls

not positive what you mean by weapon but by your example yes you can throw people into the fence. about the only thing that you can't do is hold the fence. you can pin a guy down between the mat and fence to help hold him down. with all the moves that can be done to snap your joints or knock you out. the fence is the least of your worries.