stl to mb conversion is typically searched in the context of the import of STL format to Autodesk Maya, which can be done with the use of the File ► Import function found in the latest version of the software. But in this case, it is more about reusing STL content than direct conversion.
In the past, before the support for STL format had been added to Maya, users often used a workaround that consisted of exporting STL files to OBJ in Blender and importing the OBJ file to Maya.
When it comes to a direct way to export stl to mb, perhaps some of the third-party converters may be of help here.