For all versions of Unity supported by this version of Bulk Edit you can add a new alternate extension for selected subscribers based on rules off their primary extension.  You can add/remove digits, append digits, add/subtract values or some combination of all those things to arrive at a new alternate extension to add.  Be sure to use the test button on the form to make sure the formula you are using is what you want.  

 

For Unity 4.0(4) and later you must select a specific alternate extension position to add the new alternate extension to.  This will replace the selected alternate extension if one is already there for that position.  You can also select to delete the selected alternate extension position for all selected subscribers

 

For Unity 4.0(3) and earlier the alternate extension is added to the collection for the subscriber.  If the subscriber already has 9 alternate extensions the add will be rejected.  In 4.0(3) and earlier you have the option of removing all alternate extensions for the selected users.