Welcome to Knitting Help!
Yes, you do need to make up an extra stitch and you need to have the yarn in front to purl. So really what you do is bring the yarn forward between the needles, over the right needle to the back and then around to the front again. That will give you a loop over the needle and put the yarn in the correct place to purl.
You won't be changing the ribbing pattern even if it's a K and a P that are worked together because you'll continue the columns of K2P2 that you've established on the previous rows. On the next row, you'll K2P2 over the buttonhole row. Still, it might be neater to move the next buttonhole one stitch down (work 10sts) and then move the next buttonhole one stitch up (work 8sts).