How do I add sub-roles to a single-filled role in v5?

In v5, if you're the only member of a circle, you have the ability to sub-structure your own roles.

On the role's page, when you're the only role-filler, click on the three vertical dots on the top right corner and select "Add subrole" from the drop down menu:

This will transform your role into a circle, and you will become the Circle Lead of that circle.