Help setting up a specific domain mapping set-up


I have a multisite configured as a subdirectory install with domain-mapping and multi-domains both installed and activated. No issues with this.

I have a number of sub-sites mapped: <--- <---

That's all fine and dandy.

How do I set-up the following? ----> hosted under one directory ---->

So the above would become: <--- <--- <---

Help appreciated on this one please.