How do you know that the sand you are talking about is beach sand ? Are you close to one of the great lake's ? Before you over-complicate the situation have that soil tested for it's water holding capacity.
I think that "humus layer" is the wrong term for what you want, but i know what you mean. You don't ever want a homogeneous layer in your soil profile, that can cause problem's, even if it's a ideal material. Topdressing 1/4 inch at a time will mix the sand and compost. If you want to use more material than that at one time you need to till that in. If you topdressed four time's a year with 1/4 inch of compost you would only amend the top 1-2 inch's in your soil profile, it's probably a 3-5 year process to make a difference.