Can I add another outlet to an existing load outlet?
You may need a different outlet but yes it can be done. Back and side outlets have a clamp that will hold 2 wires under each screw or 4 blacks on the hot side then 4 whites on the neutral side. Your limitation may be the size of the box and the wire size these would be needed to calculate the "box fill". Don't forget the outside receptacle will need to be a GFCI or the circuit protected by a GFCI Circuit breaker.