When running rs.initiate(), if any nodes other than the initiator node have data (but not an oplog), an error results:

"errmsg" : "couldn't initiate : member <node> has data already, cannot initiate set.  All members except initiator must be empty."

If you later use rs.add() to a node that has data (but not an oplog), then it will cause that node to initial sync and throw no errors.

If you can rs.add() a node with data in it, it follows that you should be able to have a configuration object for rs.initiate() that contains a node with data in it (and both should cause an initial sync of that node).