I want to add a subdomain to all my WordPress sites in a WP MU.
For example, I have:
example.com. When adding a new site to be:
I don't want to create a new user or WordPress Installation.
Is this possible?
Yes, with domain mapping and a compatible hosting account. There are a few good domain mapping plugins available that may work for you with a little configuration. I have used Domain Mapping pluginby Barry, as well as WordPress MU Domain Mapping. There is also the Multi-Domains plugin from wpmudev, which allows the creation of subdomains at your primary or mapped domains. Details are available at premium.wpmudev.org/project/multi-domains/ (sorry, stackexchange won't let me provide more than two hyperlinks in my answer). You can also always try visiting the WordPress Plugin Directory or hitting Google for other options.