Lookup custom field "Reporter-Email" and if a user is found update "Reporter"

[Rachael Garbee]

I'm looking to create an automation that will look at an email address in the custom field we've name "Reporter-Email" and try and match that with a user in the system. We receive inbound requests into Jira from Google Forms via a Zapier integration and b/c of this the user request isn't always a user in Jira. I want to match when possible to a Jira user but we know one won't always be found which is why we're using the custom field 
"Reporter-Email" to generate the inbound request email. Any ideas on how I would accomplish this or if it's even possible.