PF.Site/Apps/core-subscriptions/Service/Purchase/Purchase.php

(line 223)case 5:                     $iNewExpiriTime = strtotime("+3 year", $iNewExpiriTime);                     break;

(line 284)case 5:                     $aPurchases[$iKey]['type'] = _p('triennial');                     break;

If you wanted to add only a weekly subscription option then only add this to this file.

(line 223)case 5:                     $iNewExpiriTime = strtotime("+1 week", $iNewExpiriTime);                     break;

(line 284)case 5:                     $aPurchases[$iKey]['type'] = _p('week');                     break;