Pre-existing function for api gateway event?

Anyone know a way to reference an existing externally created lambda function in the Serverless functions config? Looking for something like this:

functions:
    someFunction:
        arn: arn:aws:lambda:us-east-1:...
        events:
            - http:
                ...

Thanks,
Ben

Did you get an answer to this? I definitely need it. I want to use a single existing authorizer for all of my applications. For now, I only know to set it manually after I deploy. Ick.