Current User impersonation for Connect apps

[Rick Boykin]

We have a connect app and have user impersonation enabled. Not issues there.

We have a 3rd party app where user's authenticate, outside of anything Atlassian. Within that app, I want a user to be able to see their Atlassian issues and I would like for them to see their issues using their Atlassian permissions. Seems like a perfect use case for the Connect App impersonation. However, to impersonate a user, I need their user id, which I do not have and I'm not sure how I get it. 

One thought is to use the REST API OAuth to authenticate the user outside of the connect app, then associate their Jira userId with a user within our app. I suppose I could just abandon the OAuth token and use the Connect App Impersonation. This feels a little hackish so wondering if there is a better way.