All of the dropdowns are built in a language file in custom/include/language and are stored in an array called $app_list_strings. Essentially what you would do is run your query in the language file and then use the results to build the array for that drop down list.
If you look through the existing examples you'll see something like this.
$GLOBALS['app_list_strings']['drop_down_name'] = array(
'dropdown_value'=>'Dropdown Display',
'dropdown_value2'=>'Dropdown Display2',
);
If you do the following:
$new_array = array();
while($row = $db->fetchByAssoc($result)) {
$new_array[$row['key']] = $row['value'];
}
$GLOBALS['app_list_strings']['dropdown'] = $new_array;
You'll accomplish what you need