So you wanna import an existing batch of emails into one or more of your email lists? Here's how to do it.
Head to the Engage tab, then select the Contacts tab (Admin bar > Engage > Contacts):
Click the Import Contacts button and copy / paste your emails into the Email Addresses field, with each email separated by a line break:
Choose the list(s) you want to add these contacts to, and click the Import Contacts button: