Possible to copy a circuit from 1 Logisim .circ file into another Logisim .circ file?
I know you asked this a while ago, but in case you still need the answer, yes it is possible. According to http://www.cburch.com/logisim/docs/2.1.0/guide/menu/edit.html 

"Note: Logisim's clipboard is maintained separately from the clipboard for the overall system; as a result, cut/copy/paste will not work across different applications, even including other running copies of Logisim. If, however, you have multiple projects open under the same Logisim process, then you should be able to cut/copy/paste between them."