2" pvc pipe under concrete has a hole. can you put an 1/1-2" pipe in it.

If you do then your going to create high pressure on one side ( a backup) and low pressure on the other side which can make the back up worse. So the answer is in what its for and how much volume your pushing through it.

What does this pipe service? It's possible to slip a smaller pipe into a larger one but if not a good idea if the pipe drains a washer or garbage disposal. Let me know, Tom