Yes, its possible to integrate iRefer with your existing sign-up form on your website so that your visitors/users don't have to fill out another form.
You don't have to create a new form to sign-up users to your referral campaign.
You can sign-up your existing users by passing user's email address as url querystring (?firstname.lastname@example.org) to your referral dashboard page. If campaign is setup for automagical signup then iRefer signs them up to your referral campaign and generates a unique referral and shows them the sharing widget. When they come back to this referral dashboard page, they can see their referrals status too.
Read more details here on how to set up this type of campaign.
Alternatively you can sign-up your existing users by calling iRefer API method (newSignup) after users have signed up to your form. Please check out our API documentation for more details.