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.
I agree that it looks do-able. Since both patterns are bottom up, you could do as you suggest and knit the bunting, then at some point change over to the shawl collar sweater pattern. If you're going to make this as a sleeper, you might want the top pattern to be less fitted than it is. Are you going to leave the bottom open so you can put it on and off or were you thinking of opening it up some other way?
I just realized I omitted that I'd start with a provisional cast on for the sweater and then could pick up the stitches and work down to make the bottom. Working from the bottom up and decreasing probably makes more sense, I might do it that way instead. I want to know how you decide to do it. Is this to replace the need for a blanket? My kids would have had something like that all bunched up with their legs exposed unless it were closed at the bottom which wouldn't have worked because they would have tried to walk in it and ended up falling.