I realise this is quite an old question, but just in case anyone stumbles upon it like I just did looking to solve the same question... Like ScottieB, I too couldn't make dav's answer work for me. Here's how I solved it in Excel 2013.
Create your vertical list of choices for your combo box. Not in a table (yet) not with a heading (yet, but leave space for one) just a column of whatever you need for your dropdown.
Create/Define a name for your list using regular Sheet1!$A$1:$A$2 notation.
Insert your Combo Box form control using the name you've just created for its Input Range.
So far so good. The Combo Box will work, but new options added to the bottom of the vertical list (even immediately below the named range) will not be added to the dropdown list; the named range, on which the dropdown is based, is not dynamically expanded.
Now for the good bit.
- Go back to your vertical list (make sure you clear anything you added around the named range so it's just as you left at step 2) and add a heading above it. Any text will do, it's just a label to identify your list of choices.
(Or don't! Excel will add one for you at the next step anyway, if you let it.)
- With the heading selected, use Ctrl+T or Insert | Table. Accept the range Excel finds and be sure to check the 'My tables has headers' box then click OK. You can change the name of the table if you like: it doesn't matter.
(Note: If you didn't add a header, don't check the 'My table has headers' box to have Excel create one for you.)
Now - apparently because the original named range is now part of a table definition - adding a new row to the bottom of your vertical list not only expands the table definition automatically, but also that of the original named range, Thus the new option will appear in the combo box's list of choices. Magic!