Yes you can mix 12 and 14 gauge wire with a 15 amp breaker on the circuit.
But it is not recommended because the next homeowner or electrician may be confused and put a 20 amp breaker on it again.
Confusion can be reduced if, down in the panel, you splice on a short length (pigtail) of 14 gauge wire to the incoming 12 gague wire in order to attach to the breaker. Also put a label on that circuit wire.
It would be much better to add on using 12 gague wire since there is no telling whether more electricity won't be used on that circuit in the future.
But it is not recommended because the next homeowner or electrician may be confused and put a 20 amp breaker on it again.
Confusion can be reduced if, down in the panel, you splice on a short length (pigtail) of 14 gauge wire to the incoming 12 gague wire in order to attach to the breaker. Also put a label on that circuit wire.
It would be much better to add on using 12 gague wire since there is no telling whether more electricity won't be used on that circuit in the future.
