domain mapping multiple multisite installs to a single mutisite install

Hi

I have many subdomain Multisite installs.

I am finding I am duplicating far too much and wasting a lot of costly hosting space and so I wondered about using a single multisite to host all other sites including their existing subdomains.

I am however, unsure if I will be able to migrate the subdomains of any of the external multisites to a single main multisite. e.g. site1.example1.com mapped to site sites.mainsite.com

Hope this is not confusing

Thanks

Stef