Could you just use the sweater pattern and increase as you work your way down until you have it as long and as wide as you want? It looks like the sleep sack is nothing very complicated. It looks like a very long nightgown in fact. Yeah, I think you could.
Cheating is an option. . . . Cheaters never win and winners never cheat, but smart knitters who want to retain an iota of sanity do, cheerfully. ~~Kory Stamper