Hi,
You need to overlay sites component. The path for the sites component is "/libs/wcm/core/content/sites/jcr:content/actions/selection".
You can add new option here, however its dialog so you can try adding like below code snippet.
<workflow
granite:class="aem-ajax-call"
jcr:primaryType="nt:unstructured"
sling:resourceType="granite/ui/components/coral/foundation/button"
text="Start Workflow"
title="Start Workflow"
variant="actionBar">
<granite:data
jcr:primaryType="nt:unstructured"
box-confirmation-message=""
box-failure-message=""
box-success-message=""
box-url="/bin/workflow-servlet"/>
</workflow>
We have used AJAX call to invoke, you can use any alternate or suitable for you.
Hope this helps!
Thanks!