If you are unable to find a way to add values to a code table it might be because the values are hard-coded.
You can tell if values are hard coded by going into design mode and looking at the XML of the add form. If the values were informed by a code table then, the GUID of the table would be shown. If the values are hard-coded, the XML displays the field values.
Another way to tell is viewing a page's data list and checking the field's descriptor. If a Field ID has the descriptor of "Value List", this is an indication that it is hard coded within the XML. If it was a table, then it would list the table name in the descriptor column and be a hyperlink.