In gmail, when I click 'Compose Mail' the cursor is placed in the 'To:'
textbox. The behavior that takes place in this textbox is what i'd like to
achieve in Excel. When I begin to type a contact name, a smart dropdown
list presents all the contacts that begin with the text that i have typed.
This dropdown refreshes with every keystroke I make. By default, the first
item in the dropdown is always selected. If the first item is the right
one, I can click the 'Tab' key and the contact's name is completed in the
textbox. I could have also opted to arrow up or down or use the mouse to
select a different item in the dropdown list. Once a contact is selected,
it is added to the textbox, followed by a comma, afterwhich the cursor sits,
waiting for the next contact name. They can begin the process all over
again to choose another contact.

The user does not have to choose one of the existing contacts presented in
the dropdown list. Instead they can opt to free type. Unlike the gmail
behavior above, in my spreadsheet, I'd like a valdiation msgbox that informs
the user if they have entered something that is not in the list, at which
point they are presented with the option to add it to the list of contacts.
Only once it has been added to the list will the user be allowed to proceed.

How can I do this in Excel? Is there a snap-in that allows for this complex
behavior?

Thanks!