UDT in UDT is supported. Or at least it was. I used a lot of that when my WIP was being done in Tier 1.
You need to make sure that the UDTs are defined in the order they are used. If UDT B has an element that is UDT A, then UDT A must be defined first.
It is probably a good idea to be explicit about type in the UDT definitions and not just let it default to type based on the name of the element. That makes it a bit easier for the compiler to get the sizes of everything correct.
Cheers,
Ancient Lady
AGK Community Tester and AppGameKit Master