This question already has an answer here:
I have a domain with DNS, mail and web hosting already set up, and I'd like to use Google Docs for my domain, but retain my DNS, mail and web hosting outside of Google. Is it possible to sign up for Google Apps for business and selectively use some products (ie Docs/Drive, while keeping my existing mail server. Ideally I'd be able to set a CNAME for docs on my DNS server and be done, but it seems I would need to move my nameservers to Google?
Alternatively a setting to share Google Docs from my personal firstname.lastname@example.org Docs account as email@example.com would solve this, so people I share Docs with won't see my personal email address.