Hey @user1352​! Daniel from Customer Support Engineering here 🔧 

It sounds like you want to create a custom data attribute! You can create a custom data attribute through a number of ways, including right through the UI, and the Messenger code snippet. You can read more about how to create a custom attribute this great Help Center article. Once you've done this, the field will now be populated for your users, so you can import users from a CSV file and map whatever value you like to the new custom attribute. This attribute can now be used dynamically in Outbound messages, including emails!

I hope this helps 😊