When you purchase the commercial version of Virtualmin do you get the source with it so that it can be modified?
And we encourage third party module, theme, Install Scripts and billing developments (and anything else you can think of to do with the APIs). We'll even help out, as best we can, here in the forums. And, at some point, we'll be offering third party developers spots in our shop or some other mechanism for them to offer their developments for sale, if they would be useful to other Virtualmin users.
There is a comprehensive command line and HTTP API (everything you can do from the Virtualmin portion of the GUI can be done from the command line and HTTP), the Webmin RPC interface is available to both Webmin and the Virtualmin modules, and that is available via SOAP.
In other words: Develop away!
We would recommend you try to do as much as possible via plugins and API usage rather than modifying the core code--for your own convenience. We roll out updates very very frequently. Sometimes every few days. It could be painful to have to modify a lot of those files every time you want to get the latest bugfixes. So, if you can make use of the facilities for doing things without touching the core code, you can upgrade without any trouble.
Check out the forum guidelines!
Oh, yeah, I should make it clear that Virtualmin Professional is not under an Open Source license. It is copyrighted proprietary software. But, you do receive the source, and we won't try to stop you from modifying it to suit your needs.
And, of course, Virtualmin GPL <i>is</i> under an Open Source license (the GPL v2).