Add it as a message in one of the actions of your workflow and it will appear as an attribute in the response. This is an example that sets a message telling whether the configurator user was found or not:
<TaskDefinition name='Teste' ...>
<Extension>
<WFProcess name='Teste' maxSteps='0'>
<Variable name='theReturn'/>
<Activity id='0' name='process'>
<Action id='0' application='com.waveset.session.WorkflowServices'>
<Argument name='op' value='getObject'/>
<Argument name='type' value='User'/>
<Argument name='name' value='configurator'/>
<Return to='theReturn'>
<cond>
<isTrue>
<ref>WF_ACTION_ERROR</ref>
</isTrue>
<s>HELP! I need somebody!</s>
<s>Yeah, right.</s>
</cond>
</Return>
</Action>
<Action id='1'>
<ActionResult type='message'>
<ref>theReturn</ref>
</ActionResult>
</Action>
<Transition to='end'/>
<WorkflowEditor x='187' y='95'/>
</Activity>
<Activity id='1' name='start'>
<Transition to='process'/>
<WorkflowEditor x='93' y='79'/>
</Activity>
<Activity id='2' name='end'>
<WorkflowEditor x='282' y='108'/>
</Activity>
</WFProcess>
</Extension>
<MemberObjectGroups>
<ObjectRef type='ObjectGroup' id='#ID#Top' name='Top'/>
</MemberObjectGroups>
<Properties>
<Property name='editorOriginalName' value='Teste'/>
</Properties>
</TaskDefinition>