You don't, on two levels.
Workflows are not "added to events". An event is something that is generated by an action in JIRA. Moving an issue through a workflow is one thing that fires events. You can fire custom events in workflows. It's not adding workflows to events, that's nonsense, but you could say "use custom events in workflows". To do so, edit the workflow you are interested in and look at the post-functions - one will say "fire event X", which you can change to your custom events.
On the second level though - don't edit simplified workflows. It's not supported and your users can stomp all over your changes.
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