As the question says: can a Universal Forwarder report an internal IP? It can clearly report the external IP, but that's not useful to me.
Please describe the problem you are trying to solve. Report the IP address to whom? I've seen plenty of UFs report internal IP addresses without special consideration so I'm not sure why you are seeing external addresses. Where are you seeing the IP addresses?
Can you declare what you are meaning with internal and external IPs?