cfnRoute property for adding route to CloudWAN corenetwork target

I also checked the CloudFormation documentation and could not find it.
So it is possible that it is not yet supported.

See this issue in the CloudFormation Public Roadmap - https://github.com/aws-cloudformation/cloudformation-coverage-roadmap/issues/1384. If this is what you're after, then please vote on the issue. As per the roadmap ReadMe:

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If what you're after is something different not covered by an existing issue then create one.

As it says in the above issue, it "can be worked around with a custom resource". I find that writing Lambda-backed custom resources in Python using crhelper is very quick.

This API page is related - https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_CreateRoute.html.