How to save incoming dicom files as .dcm?

Is it possible to add/set up the dicom file extension in orthanc as ‘.dcm’ instead of no extension (default behavior) ?

As explained in the Orthanc Book, you should never read the storage area of Orthanc by yourself:
http://book.orthanc-server.com/faq/orthanc-storage.html#direct-access

In your case, you could write down a copy of the received DICOM studies by using a Lua script, a sample of which is available in the Orthanc distribution:
http://book.orthanc-server.com/users/lua.html

https://bitbucket.org/sjodogne/orthanc/src/default/Resources/Samples/Lua/WriteToDisk.lua