It is not easy to extend Virtualmin in the manner that you have described. That is to say that there are no built in options in Virtualmin to, for example, extend storage with just one click. Your idea of extending Virtualmin by getting one instance to control another is possible, as @jimr1 has confirmed, but even if you use Webmin clustering for this, the new Virtualmin server will not inherit the Virtualmin setup from the old server - you will have to setup and configure the new server by applying the same effort that you had put into getting the old server setup the way you like it to be.
An alternate strategy that you could consider is to:
- create an image or snapshot of the old server
- store the image or snapshot somewhere on the cloud
- upgrade the old server with more storage, ram and cores as required - ideally this can be done and you get to retain old IP address, on most vps service providers
- restore the image or snapshot on the upgraded machine.
If the IP address and OS is the same, the restore of the image / snapshot should work without any reconfiguration. You will have a Virtualmin system with expanded storage and resources but with all your preferred settings carried over from the old server to the new / upgraded server.
This sort of thing could also be termed ‘extending’ but it is different from what you imagined it to be.