Yes, you can treat them the same way you would T-Shirts for a T-Shirt quilt. Use a light weight iron on non woven or mesh (tricot) where the knit of the interfacing is at right angles to the knit of the top.
You can also add a coordinating fabric. No problem mixing the knit with the quilting cotton. I wouldn't use a really detailed pattern, maybe square in a square, boxed square, pinwheel etc.
So sorry for your loss.