I have done this on a crocheted bunting, actually quite a few of them years ago. I have also done this with a separating zipper on a sweater, and a large zipper on a crocheted duffle bag. I just used the zipper, no additional ribbon. Works fine but make sure you check the tension of your stitches first and you should be OK.

If it was me, I would lightly baste the zipper in by hand first. The pressure, even tho you would lighten in, of the foot along the crochet/knit material part, will stretch it out, and you will be at a loss to recoup the real look of the garment. I would also use paper under the pressure foot to help eliminate the stretch of the crochet/knit.
Good luck, hope it works out for ya. All that work, and it could be ruined quickly by a hurried mistake.

Shaddy's post could have been written by me. I did exactly as she has done. I used tissue paper (the kind you use for gifts) and then just tore it away after I finished. And as much as I hate to baste, I found it was extremely helpful to do that, too.

Thanks, ladies. I always wondered how to do this best.

Thank you for your advise and support. I will use a stabilizer. I feel a little braver now that I know some of you have used a machine to put the zipper in garments. I think it will be stronger and hold up to tugging it closed.

I forgot to mention that I used a longer machine stitch to put the zipper in, almost (but not quite) as long as a basting stitch (my machine is a 4). For a regular garment that wouldn't hold a seam well, but in a knitted or crocheted fabric the smaller stitch didn't work as well as the larger ones.

if you have time and yarn left, knit a small piece and test out the size stitch/tension that would work best

Also, do you have a walking foot for your machine? If you do, USE IT! It will evenly feed top and bottom with out stretching the knit fabric so much.