Save .nrrd file into STL file

If your nrrd is binary (i.e. contains a segmentation) then you can load it as a labelmap (click Show Options in the load window and check LabelMap option). Then go to Data module, right-click the labelmap, select Convert labelmap to segmentation node. Then go to the Segmentations module, select the segmentation, go to the very bottom to the Export to files section, then select your options and click Export.