Colorize triangles and let Netfabb derive the new face group
Use
Texture & Color Mesh to set colors to triangles. Triangles with the same color will, after conversion, belong to the same face group.
This uses the colors to set or add information to each of the triangles about what face group it belongs to. To clear all face groups (equivalent to setting all triangles to belong to the same group), perform the same steps, but coloring the entire part in one single color.