Hi. May be my solution is little outdate. But for next generations, i want to make reply.
So, your question can be devided to two parts: find some area and make some rules to navigate in this area.
So. First part can be resolved by custom Services in Behaviour Tree. By some perception sense NPC can scan level to search areas by custom rules(for example sort objectives by priority).
Second part, when area was choosen, you need to generate some NPC navigation. You can use EQS system. For this, you need to generate points in this area, by using custom EQS context, then you can filter generated poins by using EQS test named Overlap. I had same question and make it resolve by myself, what printed in another topic: AI moving in specific area. .