The ability to import/adopt resources into an existing CloudFormation stack is the #1 ask from CloudFormation customers. We've been thinking about ways to do it for a while, but haven't hit upon the mechanism that both fits customer needs and works at the scale the service operates.
Since we don't expose stack state info anywhere outside the service for you to modify, the only approach you can take until we offer an adoption feature is to either store metadata about the resources in a parameter store, or use a custom resource as a wrapper to retrieve the information about the underlying resource and then surface it to your stack via Fn::GetAtt.
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