It is possible, but not through the UI:
The only difference between a Snippet Folder and a regular Library Folder is that the default action is "Insert Source" and not "Open". There is nothing special about the files.
We did not expose this in the UI, but you can specify it in the Registry by adding a DWORD called DefaultAction and setting it to 1.
"HKEY_CURRENT_USER\Software\XLnow\OnScript\OSEditor\Libraries\OnScript Snippets" is where you can find the entries for the Snippet Folder we supply. Apply the same to your Library Folder and you are set.
XLnow - the makers of OnScript