You don't have to add anything if you are using this method.
Let me see if I can find a tutorial on how to fold the backing over to the front, that shows how you will end up with a built in pocket :D:D:D
I must have lost this tutorial when my computer crashed...
Donna Dewberry uses it in her One Stitch Quilts.
You fold the backing fabric on all 4 corners to meet the quilt top.
Then you fold the backing again top, bottom and then sides so that the edges just touch the top.
Then you fold it again so that it comes over the top of the quilt top.
Stitch down the folded edges, you can sew up the other openings if you like.
You end up with a sleeve going around all of the edges, you can see in the picture the built in sleeve.
I made this one a bit different, and the corners are more open that what yours may be.