If not can this be added as a feature request. It appears this was also raised about 5 years ago here  http://forums.manageengine.com/topic/email-address-selection with no response from the ServiceDesk team.

The main issues we have are

• The drop down selection contains AD users and customers so the list excessively large.
• There is a very high risk of a technician accidentally selecting a customer instead of an internal member of staff! Some serious data protection issues here! 

At the very least there should be a button to press to bring up an address book to select addresses rather than auto completed.