You can import your existing users into your iRefer campaign using Zapier. This will create a unique referral link for each of your current users and also send them a welcome email with your campaign details and sharing options. You can switch off this welcome email if required.
Here are some examples of the different applications you can import existing customers from.
Create referrers in iRefer from new spreadsheet rows in Google Sheets
Add new users in ActiveCampaign to iRefer
Add new subscribers in MailChimp to iRefer
Add new subscribers in ConvertKit to iRefer
Add new subscribers in Drip to iRefer