I’ve been working on the main collisions of my multiple vehicle crash simulation, but now I want to add an additional vehicle to my EDSMAC4 event? Is this possible?

No. When you first created the event, you selected all of the objects that would be allowed in the event. You cannot add or remove any vehicles from the event. However, you have two possible choices to consider. One is to recreate your entire event and add the additional vehicle initially. The other is to add an additional event with only the new vehicle and then combine the Traj Sim sequences together in the Playback Window to complete your sequence.