Pretty basic question. We have an existing swarm and I want to start migrating to Kubernetes. Can I run both using the same docker hosts?
See the official documentation for Docker for Mac at https://docs.docker.com/docker-for-mac/kubernetes/ stating:
When Kubernetes support is enabled, you can deploy your workloads, in parallel, on Kubernetes, Swarm, and as standalone containers. Enabling or disabling the Kubernetes server does not affect your other workloads.
So: yes, both should be able to run in parallel.
If you're using Docker on Linux you won't have the convenient tools available like in Docker for Mac/Windows, but both orchestrators should still be able to run in parallel without further issues. On system level, details like e.g. ports on a network interface are still shared resources, so they cannot be bound by different orchestrators.