Well, although I personally would not make them one agreement, you can certainly do so if you would like. You can add language like this to the sales contract:
"Beginning <insert date>, Buyer shall be deemed to be in possession of the Property and will pay a monthly fee of $X.XX until Closing. Closing shall occur on or before May 1, 2016. This provision shall not be deemed as a residential lease, Buyer shall not be deemed as a tenant, and there shall be no requirement to evict the Buyer in the event of nonpayment."
Here are some problems as I see them: (1) Even with the above language about it not being a lease, I don't know how you would realistically get the buyer out of the house without an eviction if he fails to pay rent. I totally understand that you don't want a lengthy eviction. But if he doesn't pay, then what will you do? If you try to physically remove him, he'll likely contact the police, and the police will not likely make a judgment call in your favor. They'll likely look at the sales contract, see that he has the right to possess the house, and see that he already is in possession of the house, and then tell you to get out and take it up on court (which means filing the eviction). (2) What if the buyer damages the house? Can you get rid of him? What if his answer is "hey, I'm buying this house so you shouldn't worry about it"? What law would you use to get rid of him and protect the house? If he is a tenant, then you can rely on landlord/tenant laws. Things are certainly more complicated if he's not a tenant.
But I'm not trying to dissuade you if you think that it's best to go that route. I'm just trying to point out where problems could arise.