go to wp-includes\js\tinymce
and pick up: tiny_mce_config.php
search in that page for: (ctl-f with most notepads/editors)
$mce_buttons = apply_filters(
When you find that line go to where the word 'bold' is, and add your feature ('hr') etc etc there just like the rest of the words are. Also, use 'separator' to make one of those fancy vertical bars in the editor tool bar.
if you want to see all the possible buttons that you can have (without adding plugins) go here: