Maybe not. It depends what the senate by-laws say. I can't find a copy to see if the post is just restricted to sitting senators. Obviously appointing a non senator to the post would n't make the person a senator ,but there might be a loophole there that would not preclude the post going to a non senator.
Can anyone find the senates qualifications for the position? - Tom
Senate Majority Leader is know as the first among equals. He has to be a senator. In the House, the Speaker does NOT have to be a sitting house member but I dont know of any case during my lifetime when the speaker hasnt been a House member.