You can either:
Order new foam pads from the dealer like I did (about $90 a peice). If you go this route, make sure to go back to the junkyard and get the interior trim code of the Explorer. You can find it on the drivers door build tag. The reason for this is that the Explorer came with a wide variety of seat options and they will need the code to order the correct shaped foam pad for the exact seats you have.
You can get an upholstery shop to recondition, add or replace the foam. I'm not sure what they charge to do this, but I'm sure it's not very cheap.
'03 XLT/FX4 L1 4x4 4.0 Arizona Beige Automatic (my weekend ride). '16 Ford Escape Titanium ecoboost (wifes ride) '16 Ford Escape S (my work ride)