You need to use a JavaScript Option object to dynamically add new options to a select box. 

For example, the following code will add a second option to a select box, or replace an existing one if it’s already there. 

document.forms[0].testselect.options[1]= new Option (’new text’, ‘new value’);

where new text is the text the user sees and new value is the VALUE of the new option.

From the excellent quirksmode site