FM support only 3 databases (MS-SQL, Oracle, MySQL) for external data source. MongoDB is not in them.
I know zilch about MonGoDB, but I'm pretty sure there exists command line tools to query it.
If that's the case, then you can use Base Element Plug-In (best plug-in on earth BTW) BE_ExecuteSystemCommand.
That would output text you'll be able to import.
I did so with MySQL, avoiding the use of an ODBC driver