You can use hook_form_alter to alter the menu link edit form. 
This way you can add a extra field to the form, in your case:

$form['subtitle'] = array(
 '#type'=&gt;'textfield',
 '#title'=&gt; t('Subtitle'),
);


Then add an extra submit hanlder to the form:

$form['#submit'][] = '_my_submit_handler';


In your submit handler you can access your custom value by using $form_state['values']['subtitle'].

You will have to store this value in an extra sql table or a variable_set,together with the menu-item ID.

Once you have done that, in your theme function you can access your subtitle by menu id.
Or you can use  MYTHEME_preprocess_menu_link to add extra info to $variables, which you then can access in your theme function.

Hopes this helpes you out.

How to add custom fields to menu items
On Windows 7 Ultimate, is there a way to see when I logged on into the current session?

I want to find out how long I have been at the PC / when I started it up.
