There are quilts already made to be either top or under quilts. Most dedicated under quilts with differential cut baffles, or other special features maybe intended to cut weight are going to be too short or too narrow for ground use. I would make synthetic panels and sew on strips of double sided Velcro (omni tape) to connect them to the under quilt to increase width, rather than do any permanent mod to the uq.

Look at jacks r better for examples of multi use quilts. I have two.