This question looks a little neglected. The short answer is - you can't. However - never let the limitations of one control dictate what you do with your site.
You can have both a plain text input field and the drop-down box positioned such that it is fairly obvious to the user that if they wish to enter a different value, they put it in the textbox (perhaps have an 'Other...' option in the drop-down which would then set the text box to enabled/visible)
You can then use javascript to set the value of a hidden field based on the contents of the two controls. Ie if the value of the dropdown is 'Other...' use whatever is in the text-box, otherwise use what is selected in the drop-down.
Hope this gives you something to work with.