Attach functions to externally defined API Gateway?

Is it possible to use serverless to attach the functions defined in serverless.yml to a API Gateway defined outside of the framework by referencing either ARN or ID?

Short answer: No.

Longer answer: A Serverless Service comprises of the Functions and their event sources (i.e. API GW), all of which are managed in a CFN template. I imagine CFN will support “adopting” existing resources at some stage, but it is not possible at the moment.