It is not possible to create a Cloud Composer environment using an existing GKE cluster. This is because the cluster is created as part of the environment provisioning process, which associates that specific cluster with the environment (and also labels resources, etc).

This association is permanent: if you accidentally delete a Composer environment's GKE cluster, you will need to recreate the entire environment. Therefore, rather than associating a Composer environment with a GKE cluster, you should probably just create a new environment and modify the GKE cluster as you deem necessary.