Customize The Signup Process Form and Email To Admin

Hello - we are wanting to capture a "How Did You Hear About us" field on the wordpress multisite signup page to be able to track the signups on our website. In turn, when someone fills out that field, we want the new site email that is sent to the admin account where it has the new site and user information, to populate that field in the email. Can this be done and where can we edit the admin notification email? Thanks!