Don't try to include the logic into the standard class. Just create your own class ZCL_LEAVE_REQ (or whatever), create one static method REJECT, copy the code/idea from the approval method, and add it to your own method. Then just use this new method.
Regards,
Karri
PS. You need to add the interface IF_WORKFLOW to ZCL_LEAVE_REQ in order to use it in workflow.
PS2. And of course you need to modify the method a bit. Now it is referring to the request id with me->req_id. You need to give the request ID as a input parameter for the method instead of referring to the instance.