Never mind, I got over it! Actually I wasn't looking for really putting something on the wp_footer() but enclosing a wrapping div aroud the thematic div#footer which isn't even located on wp_footer(). My mistake =)
I figured the only way is actually using 1thematic_abovefooter()andthe thematic_belowfooter()`
I did it with two very simple functions:
// Open #footer-wrapper last in thematic_abovefooter()
function footer_wrapper_open() {
echo '<div id="footer-wrapper">';
}
add_action('thematic_abovefooter','footer_wrapper_open',20);
// Close #footer-wrapper first in thematic_belowfooter()
function footer_wrapper_close() {
echo '</div><!-- #footer-wrapper -->';
}
add_action('thematic_belowfooter','footer_wrapper_close',1);