You can now use AWS CloudFormation templates to automate the deployment of solutions that use AWS Transfer for SFTP (AWS SFTP) servers. You can now also upload and associate an RSA host key with your AWS SFTP server.  

from Recent Announcements https://aws.amazon.com/about-aws/whats-new/2019/05/aws-transfer-sftp-now-supports-aws-cloudformation-host-key-import/