Can my orthodontist put a wire at the back of my pontic so it won't move as much ? the pontic is currently attached to a orthodontic wire to a bracket

Possibly: Depends on how much room there is and which tooth you are talking about. Why not ask your orthodontist?

Possibly: But only if this does not interfere with tooth movement. The only professional who will know if this is a viable option for you is the one that knows your personal circumstance best, your Orthodontist. So at your next appointment, ask.

Maybe: There are many ways to stabilize the plastic tooth. Your orthodontist will choose one that doesn't interfere with treatment.

They might: but there may be reasons that they did not already. Best to ask this question directly to your orthodontist.

Probably not: I presume you are wondering if they can attach the Pontic to the neighboring teeth with an additional wire on the back side. The problem with that is that the orthodontic treatment is moving your teeth. If they attach the Pontic the way you suggest it will prevent these teeth from moving and impede your orthodontic treatment. They may be able to use a different bracket to grip the wire better.