Hi --
Is your drop down list populated from a secondary data source? Because, basically, you'll need to submit any 'new' items to that data source, then refresh it for them to show up in the current form.
If your form itself is bound to a DB and you submit info from it to a DB, another option that may be workable for you is to use the values submitted already to your table for that field as the source of your drop down. Provide the user with a drop down control and a text box (maybe a cool looking button to allow them to chose what to display, you can make it look snazzy). If the option doesn't exist in the drop down, they use the text box. After they submit their form, the new value is now in your main table. Since it is feeding your drop down as well, as a secondary data source... the next time the form template is open, the new item will be an option.