Gmail – Send Email to Google Circle or Create Contact Group

Yes. Add it the same way you'd add any other email to send from:

1. Click on Settings in the upper right.
2. Click on Accounts and Import.
3. In the "Send mail as" section click "Add another email address you own".
4. Add the "plus email" and finish the setup.
5. Compose an email and select the custom "plus email" in the From dropdown.

There is not an explicit way of doing that, but it is a realy fantastic idea that you should sugest to the Gmail team.

http://mail.google.com/support/bin/static.py?page=suggestions.cs&issue=116880&bucket=16937

I work in the same way with both emails is Outlook and you just need to get into the habit of checking the "From" address before you click send. Enabling the "Undo" lab goes a long way to rectifying mistakes.