How would you like to use existing secrets?
If you want to refer to another resource, use the resource's ARN or ID.
When referencing resources in CloudFormation, parameters are simply obtained and used according to the specification of each resource.
For example, AWS::SecretsManager::Secret will get the ARN by "Ref".
By using a custom function, I think that it is possible to create something that obtains an ARN string from a resource name that is not managed by the stack.
If you want to manage it as a CloudFormation stack, consider incorporating existing resources into the CloudFormation stack by referring to the following.
How can I build a CloudFormation secret out of another secret?Accepted Answerasked 7 months ago
How do I restrict AWS secret by Lambda alias?Accepted Answerasked 7 months ago
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Can CloudFormation add existing policy to existing role?Accepted AnswerMODERATORasked 2 years ago
Error retrieving a secret value from secrets managerasked 7 months ago
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Can I rotate my secret every hour?asked 3 years ago