I totally understand that Backstitches/Fractional Stitches are critical part of your design. Unfortunately, you can't use them with current StitchSketch (as of ver. 1.62).
Right now, I'm working hard to implement it, and getting there - so please wait a little bit more!
The development is in-progress!
"Backstitches/Fractional Stitches" will be available only on "Full"(paid) version.