With the Bulk Actions feature, adding existing Subscribers to a Sequence is easier than ever!
On your Subscriber page, you can 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 to Sequence' from the dropdown menu.
Select your Sequence from the menu.
Hit 'Save' and Voila! Your Subscribers are now added to your Sequence.
Note: This date is now their "date of subscription."