Method to override ImportValue variables

[pflugs30]

Is it possible to mock or override CloudFormation functions such as Fn::ImportValue or Fn::GetAtt while using serverless-offline? I'm able to override variables when using the plugin serverless-appsync-simulator (see this documentation). In particular, I'd like to override an ImportValue reference to a GraphQL endpoint with a local url to simulate the whole app stack. Thanks!