In order to add a referrer who isn't an agent on your roster, the best practice is to add them as an Affiliate. Affiliates have the ability to refer, track referrals, etc., without being an agent on your team.
From the Retention screen, click Add Team Member. Enter the referrer's information, and under Role, select Affiliate.
Not only does this give you the option to enter referrals under their name, it gives them the ability to track progress and easily refer you more leads.