On Webmin > Servers > Apache Webserver > Existing virtual hosts > Virtual Server domB.com Port 80 > Edit Directives
I added the following lines:
ServerAlias domBX.com
ServerAlias domBY.com
You could also just add an alias to Virtualmin, rather than manually changing the Apache config. You can do that in Create Virtual Server, and click the “Alias of …” link.
Prior to doing that, if you want aliases to redirect to the parent, what you can do is go into System Settings -> Server Templates -> Default -> Apache Website, and on that screen, set “Create alias websites by” to “Create permanent Redirect virtual host”.
It defaults to just adding a ServerAlias option to the Apache config, which doesn’t redirect.