The great news is, Bill has 2 books full of his artwork and techniques for getting that yummy texture in his The Pastel Book and The Oil Painting Book, both published in the 1990's and still readily available.
The trick he shares in The Pastel Book about how he creates his layered and textured panels for painting with pastels and oils is invaluable if you want to create similar airy artwork.
I have found that besides the type of acrylic modeling paste Bill recommends in his book, I have had success with Golden's Coarse Molding Paste and Pebeo's High Density Modeling Paste as well. If you love textured surfaces and artwork like I do, give Bill Creevy's panel making trick a try!
Happy Arting! :D Tristina