"AppTierS3AccessRole": {
        "Type": "AWS::IAM::Role",
        "Properties": {
            "AssumeRolePolicyDocument": {
                "Statement": [
                    {
                        "Effect": "Allow",
                        "Principal": {
                            "Service": [ "ec2.amazonaws.com" ]
                        },
                        "Action": [ "sts:AssumeRole" ]
                    }
                ]
            },
            "Path": "/",
            "Policies": [ { "Ref": "existing-policy" } ]
        }
    },

ecrRole:
    Type: AWS::IAM::Role
    Properties:
      Path: "/"
      ManagedPolicyArns:
        - "arn:aws:iam::aws:policy/AmazonEC2ContainerRegistryReadOnly"
      AssumeRolePolicyDocument:
        Version: '2012-10-17'
        Statement:
        - Effect: Allow
          Principal:
            Service:
            - ec2.amazonaws.com
          Action:
          - sts:AssumeRole

"ManagedPolicy": {
  "Type": "AWS::IAM::ManagedPolicy",
  "Properties": {
    "Description": "something descriptive",
    "Groups": [ ... ref(s) for groups ... ],
    "Roles: [{"Ref":"AppTierS3AccessRole"}],
    "Users": [ ... ref(s) for users ... ],
    "PolicyDocument": {
      "Version": "2012-10-17",
      "Statement": [
        ...
      ]
    }
  }
}