I need to send payment page links to user on schedule basis, is there any way to use existing scheduled email functionality trough custom extension?
you need to create an API in order to use schedule jobs
look for table called civicrm_job and make an similar entry for the newly created API and this will create an entry in schedule jobs
you can create an Extension first and then API in the Extension find the below link http://wiki.civicrm.org/confluence/display/CRMDOC/Create+a+Module+Extension
if you need more help please let me know