how to get list of all buttons in civicrm hook_buildform?
You will need to over-ride the button to include existing + new button through buildform. The other option is to add button using smarty in tpl file using below snippet

{crmButton p='civicrm/request' q="action=browse&amp;reset=1&amp;gid=$gid" icon="th-list"}{ts}Request for Approval{/ts}{/crmButton}


HTH

Pradeep