I have SewWhatPro, which now has that function. Is an all over stitch that flattens the nap. It will create itself a bit larger than your design. I've used it on towels.

You could accomplish the same by placing a piece of Tull on top of the material before you start embroidering. The Tull tears away very easy after you are finished and keeps the nap down.

Sorry to intrude
I cant find a design. And dont know how to seatch on this site

Far easier (on towelling) to just iron the area of the design, with spray on starch, being sure to iron in the one direction that gives the flattest result.
By doing this you get a flatter area on which to stitch without the added bulk of additional underlay stitching.
Just my opinion, but it is the way I always embroider on towelling. I will also add tearaway stabiliser, or dry cover up, on top of the towelling to help prevent pokies coming through the embroidery down the track.

If I just want to add support to prevent stitches sinking into fabric, I will use a layer of WSS on top instead of tearaway or cover up.

Everyone has a technique they prefer, you need to experiment until you find what works best for you.

Some designs have no underlay at all, when they should have, but short of digitising a universal underlay for such designs I don't know how you would do 'knock down stitches' to match each design just to fatten nap, or replace missing underlay.

Sorry it's not much help. :-(