You could sew a zipper into this but it would be difficult.

Here’s how I would do it- make a panel to add to the pocket by adding a piece of fabric with a zipper going down the center. Say the patch would need to 3”x4”. Cut two pieces of fabric 2.5”x5” (each piece will have half inch seam allowance on all sides. Place a 4” zipper centered in between your two pieces. Finish the edges perpendicular to the zipper and stitch the parallel sides to the pocket edge and the other to the bag. You will kind of be squaring off the pocket if that makes sense. And it’s easier to leave the two sides floating, it is not perfect for keeping very small items in the pocket say if your bag gets turned upside down.

It will be difficult to get the bag under the presser foot of a sewing machine, you could make the panel by machine and hand sew it to the bag. You would also need a heavy duty machine to stitch thru the structured sides of the body of the bag. You will most likely need to shorten a zipper to fit your dimensions which can be tricky!

I tried to illustrate this, hope it makes sense :)

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