I can't tell you exactly how to insert your code because it may have things in it that would change the way it needs to be inserted. You can try using the code below, but remember that if there is an error, you will need to use ftp to correct it.
Add this under the
global $post; line:
your extra code replaces this line
Be sure that the 'EOTEXT;' line starts at the left margin. It won't work otherwise.
Then, insert this code as explained below:
if ($post->$post_type == 'post' || $post->post_type == 'page') $text .= $extra_text;
If you want your extra text before your signature, insert that line before the existing
if($post->post_type == 'post') line.
If you want the extra text after the sig, put that line just before the 'return $text;' line.
If you would like to be sure that this will not cause errors, put the code for the extra text in a pastebin and post a link to it here and I will check it for you.