None of the comboboxes in GDA are limited to the picklist provided, that is, you can always write in your own value. However, unless noted, that value will not be stored permanently with the picklist, only within the attribute field that the combobox is bound to. To permanently add the value to a picklist that is not editable on-the-fly, you will need to edit either the .dbf file or the .apl file, depending on where the list is stored.
If the picklist is stored in a .dbf file, use a .dbf editor (Access works best, Excel works with mixed results, or google for CDBF) to make changes. To sort the picklist in a particular way, add a field and add values that when sorted, in Excel or Access, will result in the list reading the way you want and then save the layout of the table. Look at GrainSizes_0.5.dbf as an example.
To edit an .apl file, open the file in a text editor (make a backup copy first!) and search for the line <LISTITEM value="choice1" text=" choice1"></LISTITEM>, where “choice1” is the first choice listed in the instructions for the particular combobox (see the help page for the page the combobox is on.) Follow the syntax in the .apl to enter new choices. The ‘text’ is the choice the user will see when the list is displayed and the ‘value’ is what is stored in the attribute table.