How to apply Serverless to an existing Lambda

When I try deploy to an existing lambda function configured in serverless.yml as following, it says “An error occurred: ApiLambdaFunction - an-existing-function-name-created-by-my-devops already exists.”

functions:
  api:
    name: an-existing-function-name-created-by-my-devops

So it is not allowed to deploy to an existing lambda not created by serverless?