You can do this with a data validation dropdown list and programming.
There's an example he

http://www.contextures.com/excelfiles.html

Under Data Validation, look for 'DV0017 - Select Multiple Items from
Dropdown List'


L2Lafferty wrote:
Can a drop down box be created to allow for multiple selections within the
list.
Example: Sales, Finance, HR, R&D, Purchasing. Can Sales, HR and Purchasing
be selected.


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Debra Dalgleish
Contextures
http://www.contextures.com/tiptech.html