I have never messed with any radomization, but I would see if you could create a location, then create a trigger that makes a certain unit of a computer player anywhere every so many miliseconds, then tell it to move the location to that computer's units (it could be cloaked so you wouldn't see it) Then just have that location make the number of units you would like. You would have to make sure the computer unit your centering the location on doesn't attack or move if it is cloaked, you might be able to make it one of your units you are wanting to create.
Once again this is just something I'd test I've never done it and I'm not sure if it will work. Their is may be a simpler way, I just don't know it. -If someone post after me that this is a dumb idea go with what they say
ADDITION:
You could junkyard dog that cloaked I was talking about instead of killing and creating it after so many miliseconds.