I invite you to find the "Digester Creep Spray" ability to see how they did (assuming you have it in your dependency). Most to keep in mind, is to make a dummy unit (invisible) that has a creep behavior. Now there are ways to keep control of that unit, but we can manage that later. There's also the possibiliy of having a create persistent effect use a creep effect, at a target location point, but remember, having a lof of those will lag your map like crazy. So it's up to you, what method would you like ?