I haven't sewn velcro onto any knitting yet, but I did just put a zipper into a knitted purse, so here is my experience. The feed dogs did have a bit
of snagging (a bulky weight, wool-blend yarn), but nothing tragic. I just went slow and careful. As long as I sewed with the zipper part facing upwards, I had minimal trouble...it was once I turned my knitting so the knitted fabric was facing up that the presserfoot gave me more trouble with the fabric catching than the feed dogs did. My machine is made to go over many layers of denim, so I figured that some bulky weight knitting fabric shouldn't have been a problem for it. The only other things I'd suggest to you to try are to make sure you have thicker sewing needles maybe (I used a size 100/16 one made for jeans/denim because the yarn I used was bulky, coupled with a zipper), and you may have to adjust your stitch length/tension (whenever I have thick sewing to do, I have to lengthen my stitch length and loosen my tension a bit). And although this may sound silly, make sure you use a brand new sewing needle on your machine--I always forget to do this (I guess I think they're supposed to last forever!
), and I inevitably always have trouble with my sewing. It's a small thing I should do first and foremost to avoid trouble since they get dull so fast.
I also use a product called Sewer's Aid
, which is a really great product that helps make sewing go easier. There are different ways you can use it; I use just a drop on my sewing needle, then I sew for a minute or two on a piece of scrap fabric before sewing on my project to make sure there's no residue (I've heard you can also put a few drops onto your thread spool and/or bobbin spool thread, but I haven't tried those techniques). It makes the thread glide through the needle and the needle really work easier through your fabric, and it helps with skipped stitches. You can scroll down a bit on this
page and read a bit more about it.
Good luck, and I hope it turns out great for you!