Grouping subdirectories under a custom subdomain


I am wondering how to group all subdirectory sites under a custom subdomain. To explain simply,

I would install MultiSite on and allow new sites to be registered under subdirectories, but instead of the URLs being, say I want the URLs to be

An easy solution would be to install MultiSite on itself and have another WP install on, but this would mean that the users would have to maintain two usernames and separate logins for both.

So is there a simple option to have the sites grouped under a custom subdomain, or have the usernames merged and have them the same for two different WP installs?

Thanks a ton. :slight_smile: