There's no way to do that with just Gmail. Gmail requires that an email address be validated before it will let you automatically forward messages to it. This is an anti-spam measure and, while it might be inconvenient to some, I shudder to think what my inbox would look like if it didn't exist.
You will need to find some other tool to accomplish your goal.
In the page where you specify the outside mail link, you can also specify a label to attach to all the email, you can also specify that you want it to skip the inbox.
To make it easy to see those emails you can then use the multiple inboxes. You enable that through settings/advanced; then you are given a new tab in the settings, and can specify the label ...