I'm afraid there isn't a way to do this in the current public build. However, this functionality is coming soon! In the BETA build, EAS has been added to User Defaults, so System Administrators can set the default status for new users as well as propagate any changes to all users. Domain Administrators will also have the ability to select all users in their Users list and use the Actions button to Enable/Disable EAS across the board.
In the meantime, as the System Admin in the Domains grid, you can use the Actions button to export a list of all of your users (Export All Users to CSV). This might make it a little easier, allowing you to copy/paste the email address in the ActiveSync field.
You can also find the mailboxes that have EAS assigned in the EAS.json file. (Its default location is C:\Program Files (x86)\SmarterTools\SmarterMail\Service\Settings\eas.json.) If you find it easier, you can stop the SmarterMail service and add the email addresses directly to this file. Just be sure they're entered in the following format:
I hope this helps!