I was also wondering about this. I don't think there's any stretchers, or animations that resemble people carrying someone on a stretcher, but doing a med evac wouldn't be to difficult.
Keep in mind, i'm very green at mission building and scripting, but if I were to do it I would do it this way. Make a script that if one of your soldiers gets killed, revive him but set his damage to just before death. Make him drop his weapon and adjust whatever other behaviours you want (he's wounded, so he's not going to be engaging the enemy). Add all wounded soldiers to a group, and then script a helo, or an ambulance to come to the area and collect all the soldiers in the wounded group.
That's what I would do. I don't know if there's better ways or not though.