Yes, You can import them by email address only if you like to a designated list, then select all members, do a data value change and change unsubscribe to true. I would typically do this again and change the unsubscribe reason as well and note where the list came from and why the change.
When importing data into MKTO, and you have an unsubscribe column, will MKTO understand True/False, or Y/N as data in that field? Or is it only 1/Blank?