Adding Reject option for the reviewers

I am facing some blockers with the following implementation :
-> Once I start the Review Task and assign the topic to be reviewed by a designated reviewer, the reviewer should also have an option to reject the task (in that case an email will be triggered to the author who has initiated the workflow that the item has been rejected by the reviewer along with the comments that the reviewer has put in).

Customizing the toolbar in the inbox section is not possible in AEMaaCS is what I have come across - so I am looking into some insights as to how we can modify the ReviewDITAMAP workflow to accordingly have our requirement fit in?

OR another idea that came across my mind was to somehow capture the comments section when the reviewer hits complete. Maybe if a comment like Rejected is put in I want to capture that and accordingly send a notification/email to the initiator stating that the workflow has been completed and when he/she opens the notification the comments should show up with "Rejected".

@divrajsingh - Can you please help out here?