Use bulk actions to easily tag your existing subscribers.
Go to Subscribers under the Grow tab in the ConvertKit navigation. Then, use the checkboxes to select your subscribers.
If you are adding all subscribers, select the checkbox at the top, and click the link "Select all XX Subscribers in your account."
NOTE: If you are adding subscribers from a specific tag, you'll want to click the tag and open that to select your subscribers.
In the upper right corner, click Bulk Actions and select Add Tag from the dropdown menu.
Select your tag(s) from the menu that pops up.
Hit Save and that's it! Your subscribers are now tagged.