Manage your Themes footer links for every blog into a multisite includes with conditional tags.
Use a Conditional Tag like
is_home() if you add the link into the
textarea field by the form.
Look if the author are allowed to change it. Someones don't like or not allowed it. If you using a free theme by
wordpress repositary in most themes you can change the footer links with your own code. Replace this footer links
with the function
<?php do_shortcode('[wplerlm]'); ?> and add some links for every blog to show your own links
on the footer of this theme.