You've to define your own outcome-contraint for in your workflow-model,e.g:
<type name="my:myTask">
<parent>wf:activitiReviewTask</parent>
<overrides>
<property name="wf:reviewOutcome">
<default>Restart Review</default>
<constraints>
<constraint name="my:myOutcomeOptions" type="LIST">
<parameter name="allowedValues">
<list>
<value>Approve</value>
<value>Restart Review</value>
<value>Reject</value>
</list>
</parameter>
</constraint>
</constraints>
</property>
</overrides>
</type>
Of course, you'll have to add your custom sequence-flows to your workflow definition.
<exclusiveGateway id="reviewDecision" name="Review Decision"></exclusiveGateway>
<sequenceFlow id="flow3" name="" sourceRef="reviewDecision" targetRef="ReviewRejected">
<conditionExpression xsi:type="tFormalExpression">${wf_reviewOutcome == 'Reject'}</conditionExpression>
</sequenceFlow>
<sequenceFlow id="flow3" name="" sourceRef="reviewDecision" targetRef="RestartReview">
<conditionExpression xsi:type="tFormalExpression">${wf_reviewOutcome == 'Restart Review'}</conditionExpression>
</sequenceFlow>
...