Ref resource from another CloudFormation without output?

Is it a way to reference a resource of another CloudFormation which is not outputted?

For example, in file1.yaml

Resource:
  SG1:
    Type: AWS::EC2::SecurityGroup
    ....

And in file2.yaml, I will need to reference SG1

  SGMSKClusterIgress6:
    Type: AWS::EC2::SecurityGroupIngress
    Properties:
      SourceSecurityGroupId: SG1 # How to reference SG1 here
      GroupId: !ImportValue MySecurityGroup
      IpProtocol: tcp
      FromPort: 443
      ToPort: 443

I know if there is Outputs in the file1.yaml, for example,

Resource:
  SG1:
    Type: AWS::EC2::SecurityGroup
    ....
Outputs:
  SG1Output:
    Value: !Ref SG1
    Export:
      Name: File1SG1

I can use it in file2.yaml as

      SourceSecurityGroupId: !ImportValue File1SG1 # SG1 output name

What if it's not outputted? Do I has to modify file1.yaml to add the Outputs section and update the provisioned product?

Yes for referencing resources in terms of CloudFormation, you should use outputs. In case that you don't have access to file1.yaml, you can create a parameter in file2.yaml where you specify security group id and reference it in resource creation