You probably could, but there is no good reason to do so. It’s certainly much more work than just starting over with a fresh OS install.
Your choices in configuring BIND, Apache, Postfix, etc. almost certainly are different from the way Virtualmin would have done it…which means you’ll either be reading a ton of docs and forum posts to figure out how to make Virtualmin work your way or you’ll end up redoing it all anyway, because it’s more work to maintain a heavily customized Virtualmin installation than one that is mostly stock and as the installer sets it up.
I strongly recommend you start from a fresh OS installation and use the install script, just as the documentation recommends. Our ability to help you with a custom deployment and manual installation is extremely limited. (Honestly, I don’t even try to help Virtualmin GPL users with a custom installation. It takes too much time to even understand their problem because it is often unique to their system and often of their own making.)
Unless you are an expert, like, seriously, years of experience, with every service you’ll be using, and have a lot of free time to learn about how Virtualmin works and are able to troubleshoot problems on your own without any help from other Virtualmin users (because your Virtualmin system will be unlike anybody else’s, in possibly subtle and confusing ways), you will save time by just starting fresh. Virtualmin handles so much more of the work, that it’s almost certainly only a few minutes of your time to re-do everything you’ve already done. You are not going to lose a lot of time to doing it over. Creating virtual hosts, DNS zones, users, etc. is stupid easy in Virtualmin…that’s its purpose.
I used to try to help people go the route you’re trying to go, and after 16 years of seeing how badly it goes for most users, and how much work they end up putting into it only to give up and start fresh, anyway, has convinced me that the only ethical thing I can do here is be really emphatic: For the sake of your own sanity and time, you should install fresh. You will almost certainly regret doing a custom manual install.