I don't have an answer for you about a program. I searched for single line machine embroidery designs. There are literally thousands of pictures out there. Not sure how many translate to an actual design. What are you interested in? Flowers, waves, clouds???
https://www.bing.com/search?q=single...gns&FORM=QSRE1
Edited to add: Did you buy applique patterns? I think they always have double sewing lines. It would be tedious, but you could forward through your design when you are sewing to just not sew the lines you don't want.