Even if you had a machine that could do this, it's going to be such a hassle trying to sew a stripe on straight from inside a tube (the unopened pant leg). I can't imagine crooked stripes looking good! Bite the bullet and open the inseams to sew the stripes on. It will be less of a hassle and give much better results.

I have just found out that the Husqvarna Viking 980Q has this feature and they are having a great sale right now. I am at a retreat and one of the ladies here works at Joann's and she told me about it. I had never heard of this option. Thanks for asking the question.

I just asked this lady about changing the stitch length and she no you can't. She has the latest machine and she tried it (she works for Husqvarna).

Absolutely right! It's much easier than buying another machine and then having to learn techniques on it and most likely still being disappointed. Pulling the stitches on readymade pants is usually fairly easy if you have a good, sharp seam ripper. You will have much less struggle keeping your ribbon flat and straight this way.

If the materials can tolerate an iron and washing, you can make the task easier for yourself by using washable school glue to baste the ribbon exactly where you want it so you won't have to pull pins as you go, which might make the ribbon wobble. You do this by gluing a little at a time and quickly pressing it with a dry iron to quickly set the glue. (Steam will make the glue not stick.) Sew the pants and then wash out the glue. I use this technique on bindings, and it's a godsend.

Another alternative worth considering is Stitch Witchery, but it does tend to stiffen the fabric a bit. I've used it occasionally over the years and have had good results with nothing letting go. You would definitely want to test it with the specific ribbon and pants fabric before doing the real project.

Final alternative: professional tailor. In spite of all my experience in sewing, I have taken chores I didn't want to do to tailors, and have been pleased and surprised at how reasonable they were, but the last time I did that was to have the naugahyde sleeves shortened on my daughter's letter jacket in high school, about 25 years ago. It's worth looking into if you're reluctant to tackle this project, especially if the pants are expensive and the job has to be perfect.

I have a DSM that sets sideways, but it sounds like it will come apart when I use it. Kind of a kerchunck, kerchunk, kerchunk. I have only tried it once. If I try it again, I will try it doing free motion quilting. It makes a loop de loop, and others according to the manual.