If you want to convert E-Mail Subscribers to WhatsApp you will need a Welcome Flow & you need to create a WA.ME-Link.
A WA.ME-Link is a shortened URL that directs people to a WhatsApp chat with a predefined message. By sending this message, people can opt-in to your WhatsApp channel.
Create your E-Mail Campaign
Create an audience in your email marketing tool and send your email subscribers an email encouraging them to sign up to you WhatsApp channel. Offer a discount or pre-access to offer a further incentive.
It's important that you offer something in WhatsApp that they can't get in your email channel.