no..its illegal.. against electrical code, to add it to existing breaker panel
NO way to isolate the new circuit
the existing panel already has a HOT BUSS
you completely disconect the romex wire for the A/C circuit
pull it out of the breaker panel, hot neutral ground
stick wire in NEW box, with NEW 20 amp circuit breaker
run new 20amp power cord from new breaker box
the NEW circuit is Now completely separate from the old panel
you can place the new box, in the wall, or in a cabinet, where ever its accessible to reset breaker