Are you talking about a Content Control? If so, there isn't an easy way that I know of but you could do it with a couple of macros that would take a bit of work to create.
An easier alternative if you aren't scared by xml is to find the relevant part in the document.xml file and edit it there. Looking at the xml to find the entries of a Dropdown Content Control shows that the list entries follow a simple enough pattern
Code:
<w:dropDownList>
<w:listItem w:value="Choose an item."/><w:listItem w:displayText="one" w:value="one"/><w:listItem w:displayText="two" w:value="two"/><w:listItem w:displayText="three" w:value="three"/><w:listItem w:displayText="four" w:value="fourish"/><w:listItem w:displayText="five" w:value="and a bit"/><w:listItem w:displayText="six" w:value="keep testing me"/>
</w:dropDownList>