There are no sub-levels on bubble. There’s the homepage and then there are other pages. That’s it.
That said, you can include a hierarchy in the way you display items in navigation, etc. that makes them seem to have a hierarchy to the user. Additionally, you could set up a page with two different “views” (e.g., example.com/about?view=overview & example.com/about?view=mission) which adds a visual hierarchy to the URL itself.
However, I recommend against optimize on the URL look (generally speaking) as most users don’t notice or care. And, the ones who do notice are usually technical enough to understand alternatives.
Use groups that show/hide based on a url parameter. These can be set-up so that all of the content is in, say, group overview or group mission so that it looks to the user as if they are 2 different pages.