Workflow condition for email marketing

I really like the automatization pipelines in email marketing. But, like you all, I never have my email lists finished or closed. So I want to start sending automated emails to a target as soon as I add it to a Target List. But there is no way to use the date a target was added to a target list as a condition. Anyone has a clue on how to pursue this?

I want to send the first message as soon as hits the Target List. A second message will be delivered 3 days later and a third one in the 9th day.