Refer outside variables in AWS SAM template

Question

I am using AWS SAM for creating some infrastructures. In the below-mentioned template, the role of the state machine is hard coded. Here, say if I want to refer this from another file say config.json. How can I achieve it.?

Resources:
  OcxDataLoadMachine:
    Type: AWS::Serverless::StateMachine
    Properties:
      Name: ocx_data_load_scripts_machine
      DefinitionUri: ./stepfunctions/ocx_data_load_scripts_machine.asl.json
      Role: "arn:aws:iam::875221978636:role/service-role/StepFunctions-postgres_fact_data_load-role-0f4a24d5"

In serverless model these data we referred data using :

${file(./config.json):IAMROLE}