STL cannot store lines, only triangles. I would recommend to process the centerline data (extract data, write in any format you need) in Python - either by writing separate scripts or modifying existing PypeS scripts.
Slicer forum hosts several other communities, such as pyRadiomics and VMTK. The author accidentally wrote to the Slicer category as well, probably because the post did not appear immediately due to moderation requirements for new users.