Use bulk actions to easily tag your existing subscribers.
On your Subscriber page, 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 Tag from the drop down menu.
Select your Tag(s) from the menu.
Hit Save and that's it! Your Subscribers are now tagged.