|You don't need a serger to lettuce edge knits. All you need is a scrap of your fabric and a few minutes to practice. :)
I set my zigzag stitch to the widest width (6 on my machine) and almost the shortest length (about .5 on my machine; buttonhole length is 0). Then position your fabric so that the hem allowance is about 1.25 times the width of your zigzag. This ensures that the fabric will be forced to "fold over" when being lettuced -- you need this little fold of fabric to achieve a good lettuce edge. Then stitch, stretching the fabric both in front of and behind the presser foot. You may need to slightly angle the fabric a little to the right as it passes under the presser foot to achieve the "fold over." You'll probably need a second pass of zigzags. For the second pass, I narrow my stitch width to about 2/3 the width of the first pass. Then stitch again, this time stretching the fabric only from behind the presser foot.
This technique may need to be fine-tuned, depending on your fabric and machine -- you may need to stretch from both behind and in front of the presser foot on both passes, etc. The important thing is to test what techniques you need on a scrap.
I have tried using my rolled hem foot, then doing a zigzag pass, and it doesn't work. For me, anyway. I haven't tried decorative thread using this technique, so if you have, please post a comment and let us know how it worked!