You can't directly convert a STL file (unstructured mesh) back to a NRRD file (regular grid), but it's possible to approximate the original segmentation by mapping the STL file back to a regular grid in our SCIRun software (http://www.scirun.org). I've attached a couple of networks for SCIRun 4 (http://www.sci.utah.edu/download/scirun) that do this, and screen captures of what each network output looks like in Seg3D. If you try this out, please let me know if you have any questions about how to use SCIRun.

Ayla