Do you have to have a zipper? My thought was to make an envelope back for the pillow. To do this, all you would need to do is add some plain fabric to the middle of the pillow back. The fabric would be tucked underneath the other quilted back piece. You could add a button or a little velcro to the center to keep it closed.
Sorry, English is not my first language and I guess I haven't explained well the problem, or I'm not understanding well the answers.
Do you mean that I must discard the quilted back of my pillow and use a plain fabric instead, and just do a normal 'envelope style' pillow?
I usually have no problems putting zippers, or buttons... or doing 'envelope style' pillows, this is not the problem. But this time I did a mistake (I quilted the back panel too) so now this is not possible to do. I can't sew a zipper unless I cut the panel in two (not even if I cut, because I wouldn't have margen for seams).
I'm really sorry for not being able to explain better...