Yes, you can do this Tommo! In your GMail settings, hit “Accounts”, then “Add another email address”. Put your special “plus” address into the list as an alias. You’ll be sent a test email to that address (which will come straight back into your Inbox): click the special link in that email. You can now send “from” your other address by using a drop-down next to the From: address when composing email. You can also optionally configure GMail to automatically prefer to reply from the address on which you receive an email, if you do this a lot.

You can use this technique to send mail “from” almost any address at which you’re able to receive mail, which is especially useful if you have multiple email accounts forwarding to the same place.