Subscribe all users to Simplenews Newsletter
Figured it out. 


Go to Mysql and output the user_field-data table as a csv. Delete all
the columns except the user emails. 
Copy all the user emails to the clipboard. 
Go to Admin  People  Simplenews 

You will see options to Mass Subscribe. 
Paste the emails on the form. Select the newsletter and wait for a
while.


If you have a long email list break it down in 500 or 1000 records at a time or the server may time out.