That's a great question, I'm glad you asked!
Currently, there is no way to first select a list and then add new contacts to it. What you can do is upload your contacts, then select the box next to the email address and click the "Manage List" option in the toolbar above and move the contacts in that manner. Take a look at the screenshot below to see where to find the "Manage List" option:
How to add contacts is another great resource to learning all there is to know about managing your contacts!
I hope that helps and thanks again for reaching out!