How can subscribers be added from a list automatically to another one after going through a flow?

Welcome to the community @Rudi

It’s not possible to automatically push contacts to a list from a flow. Contacts join a list when they subscribe to it (via a form) or are added to it manually by you, either individually or via an import.

To achieve what you want, it’s better to have a main newsletter list that triggers a single new subscriber ‘welcome’ flow.

If you don’t want all new subscribers - newsletter and pop-up enticer - to get the same discount offer, you add a custom profile property to your pop-up that identifies them as ‘discount offer = true’. Then in your welcome flow you add a conditional split on that custom profile property to send them down the correct branch.

That’s usually my preferred approach, though there are others, including using display logic to show discount content only to contacts where ‘discount offer = true’. That works well when there isn’t much diference in message content between discount v no discount apart from the offer.  

Hope that helps

Andy