Create the A10 and give him a hold waypoint somewhere out of the way. He'll need other waypoints to get him to the right place and do the right thing. Now create a trigger
Activation: Radio Alpha
Type: Switch
I'm assuming you know how to handle the radio part. Anyway, he will hang around the hold waypoint until the tirgger is fired and then come in. Read the tutes in the Ed Depot to learn more about switch triggers, which are one of the most important things in OFP editing.