You can add and remove domains if you want visitor data to persist between domains by using the field This site spans multiple domains while creating a web property .This setting lets you specify how you would like to track traffic moving between your associated subdomains or domains.Within a Company account, you can use multiple web properties to separate domains and settings, or you can put multiple domains into the same web property.
This setting doesn't allow sub domains. if adding blog.example.com is says the example.com has already been taken. Some what confusing as to what sub domains means per that documentation. Are you able to elaborate on this? Thanks