Add a leaf will work but.... The ride quality will suck. You can buy complete spring sets but you will need a few other things also. If it is leaf springs front and rear you might be able to change your shackles or add blocks....
I thought the excursion had coil springs in the front.
Adding longer shackles or lift blocks is always the cheap and not the safest way. Taller blocks creates safety issues because of longer U-bolts that are not any thicker, which can bend easier. Longer shackles in the front will cause steering issues because it will affect the camber and cause bump steer along with requiring more steering input from the driver.
The best leveling kit for a vehicle with 4 leaf springs is to get new springs. They will be arched just a little more than factory, or they will have a few more leads in the pack. More leafs in the pack doesn't always equate to a rougher ride, the thickness of the leafs along with the amount of arch is what dictates the springs rate aka stiff ride.