Hi Myilraja,
It is business process definition issue. If there is no follow-on process defined after reject, it does not make a sense. If they prefer offline activities, approver can keep the workitem in inbox and send email to contract agent to update the contract. When contract updated, old workitem is canceled and approver will get new workitem.
If you want to model the follow-on process after reject, you can copy the standard workflow template and enhance it. For UI side, you can use UI extension points to add reject button.
Please use new SAP Fiori Launchpad
Regards,
Masa
SAP Customer Experience Group - CEG