shared files are represented in the mp_SharedFiles table in the database
to move a file from one instance of Shared Files to another could be done in the database by changing the ModuleID, ModuleGuid, FolderId, FolderGuid of a row. ie find the value of those for a row that is already in the target instance and then update a row that you want to move to those values.
there is no way to do it from the UI, it would have to be done from the database
Hope that Helps,