This won’t do any harm at all. In fact, back in the '50s & '60s, before oil detergents were as good as they are now, some people used to add 1 qt. of trans fluid to their motor oil once a year just prior to an oil change, in order to clean out their engine.
But, just for an extra measure of prudence, why don’t you have your oil changed in a week or so? In the meantime, just drive normally, and don’t panic over this little incident.
ADDENDUM: Superlube??
If that is a quick oil change place, you have a much higher probability of having your engine (and transmission, and brake hydraulic system, and differential, and power steering) damaged by them than by your act of adding some ATF to the motor oil.
On a regular basis, people report on this site that their cars have sustained major damage from the rapid pace and the inexperience of the poorly trained employees at quick oil change places. We have heard about ATF or windshield washer fluid being added to the brake master cylinder, transmissions being drained instead of the crankcase (leading to a grossly overfilled engine and a “dry” transmission), improperly tightened oil filters, and other examples of mistakes that have caused major damage to cars.
As Joseph Meehan, a veteran of this site says, “Don’t go to a quick oil change place–even for directions!”.