How can I extend this concrete pad?
Assuming both existing concrete pads are stable, I see no problem adding whatever bit of a pad you need for an AC unit. Do you ever see them move independently during the spring thaw? Just leave an 1/8 to 1/4 inch expansion joint between the new concrete and the pad you are trying to match level with. Simply frame up a form with plywood cut to proper height, support the sides with a few blocks so they will stay straight, pour some sacrete and screed it smooth.  Float it after it starts to set up if you want a smoother finish.  I'm quite sure you will be fine.