There are a few ways of doing this:
1. You can have the list of Contacts in a spreadsheet file in your computer or cloud and manually import them using the Import option available in the View Contacts section or

2. You can use our Lead Subscribe API to bring your Contacts from any third-party services if you are looking for a programmatic way or

3. You can integrate other Zoho apps like Zoho CRM to sync the Contacts automatically to Zoho Marketing Automation.