Select the cell that contains the list, and choose Data > Validation
You may see a list typed in the source box. If so, you can add new
items, then click OK
If you see a range reference, e.g. =$A$1:$A$10
you can add items to that range.
Then, in the Data Validation dialog box, change the reference to include
the new rows.
There's information in Excel's Help, and here:
http://www.contextures.com/xlDataVal01.html
