digitising - No and scanning - No.
Seamly does not work like standard garment CAD programs. It is parametric and you have to define points using the various tools with associated formulas, and if you want the pattern to adapt to different sizes you need to incorporate measurements / increments from the SeamlyMe into to the formulas.
That said… I figured out a way to take an existing pattern, and using the various tools to place the grade points in the proper X-Y pos, and then using standard Wolf dress form sizes I can do a standard 2" grade up or down. I presented several patterns done this way in another thread you may find useful.