I'm a newbie and there's no way that I could modify a pattern (yet!). Maybe when I get a little better and more comfortable, but right now I really need to find a pattern for pretty much exactly what I'm looking for.
They are, though, just what you described...overalls with feet in them. I'm still searching and can't find them. I've found some sleepers, but they have long sleeves - not straps. I've found overalls, but they have cuffs - not feet.