The "externalAddress" is a system type (IPEndPoint). It's not a TNet type, so no, it's not possible to add anything to it. You're the one coding your game, so you should have some kind of a system for ports, for example your +1 convention. Ie: if you're choosing to listen to TCP port 5100, assume UDP is going to be 5101.
That, or just have the UDP port be available as a part of your server's name. For example:
You can easily do a string.Split("|") on that to get your two parts, then int.TryParse(part[1], out port) to get your port.