You can configure the workflow transitions easily through the workflow designer. To add a "button", you simply add a transition from one status to another.
You'd probably like the rejection button to appear if an issue is in the status Closed or Resolved. In that case you can add a transition (by dragging an dropping) from Closed/Resolved to Open/Todo. You can then give this transition a name, fe.: Reject.
On every transtion you can configure a Screen. You can configure what fields should be shown on this screen. Please be aware that the comment field will be shown on every screen, you should not and cannot add the comment field to a screen as it'll always be there!
If you're question is about JIRA Service Desk and you'd like this "Reject" button to be shown on the Customer Portal, you can do so pretty easily. Simply check the box in front of "Show transition in the customer portal" on the Transition configuration. For more info, see here.
Hope this helps.
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