How to use existing scheduled email function programmatically in extensions?
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