I have different people who email me about the same subject, e.g, WBR and AIB both email me about HP201. However, so far as I can see Google only allows the sub-label HP201 to be attached to one parent. Aside from making HP201 a parent itself is it possible to make it a sub label of WBR and AIB?
No. A label can only have one parent (or no parent).
You could probably make two different "HP201" labels to put under their respective parents.
Really, though, you have a use-case for not having "HP201" as a sub-label. It should be a label unto itself. That way you can easily find all of the "HP201" conversations, "WBR" conversations, and "AIB" conversations. A quick search (
label:HP201 label:WBR) will easily find the conversations with both labels.