Containers don’t have notions of “disks”. The right approach is generally:
On the host, mount the disk normally
Stop and delete the existing container
Start a new container from the same image, adding a docker run -v option that points at the additional storage
Reading the description on http://www.webmin.com/ it doesn’t look like an appropriate thing to run inside a Docker container. If you want it to, say, administer the host system’s attached disks, you need to run it as a host system process. (If you need specific hardware like “SATA external drive bay” then your process is not a good candidate for running in Docker.)