You will have a job converting from .stl or scad. It seems Open scad produces very bad .stl files which seem incompatible with many CAD systems. Ok for printing only.
Believe me I have tried and it seems most .stl files are incorrectly scaled and have surface errors. This also includes most files on Thingiverse. Your supplier is clearly having problems with the surface gaps (non manifold parts) which is why they cant measure the part volumes.
Perhaps someone on here has some remodelled parts in iges (.igs .iges) format which do go into other cad systems. It is most likely your supplier would accept these also.