Is this combo box one of Word's Content Controls, a combo box on a Word userform, or is some other application (given your mailmerge reference)?

Regarding the mailmerge aspect, it is possible to generate a mailmerge field via the code attached to a combobox, but you'd need to explain more of what you're trying to achieve before specific advice can be given - there may be better ways of achieving the desired result than what you have in mind.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]