This script was written by Larry Minton. It was originally called LoadAndSaveAll.ms.
I simply have made A FEW modifications to process .stl files.
The script imports all .stl files found in a given directory (and sub-directories, if desired), then saves them out as a .max of the root same name to the same source directory.
The import uses the most recent settings.
The default settings are shown in STLImportDefaults.jpg.
So, to run script with non-default settings, import one file manually and choose desired settings, then run the script to batch process the files.
If you need to have different import settings for different files, either group them into separate directories and process by group, or search for 'importFile' and see comment.
To turn this into a MacroScript launchable from a toolbar, select all text, then drag-n-drop it onto a toolbar. That's it!
NOTE: STL import can take a good bit of time.