Sounds ok for me. Now, i removed all components from both pcb sides, grind off the green paint and scanned both sides. With a little PixBuilder magic, i've got two black/white images of the lanes, pads and vias.
Now, how to i get them inside Fritzing to draw pads, vias and lanes above it? I do not found any function nor drag & drop to place an image on one of the layers (e.g. silkscreen layer for bottom or top would be fine for that). I hope i can underlay the image somehow and it should be locked to it does not react on clicks.