I use alfalfa/ timothy pellets go stretch minimal grain meals. If I would want to stretch my hay I would do it the same way. But give more pellets, soaked, together with whatever hard feed/supplements they're getting, during the day, and give hay for the night.
Alfalfa IS higher in calories, calcium and protein than grass hay, but does not cause laminitis. It is, in fact, lower in NSC than timothy or grass hay. It will get an already fat horse fatter and that can cause laminitis. But not because of the alfalfa, rather because of overfeeding.