Is it possible to install Virtualmin pro using Webmin to install it as a module? This is how I did it for the GPL version and it works great. I would prefer to do it this way since I already have a LAMP server with Postfix and Dovecot installed.

Of course.

RPM versions can be found here:

http://software.virtualmin.com/universal/

Deb packages can be found in subdirectories of (the usual apt-get tree structure, so you have to dig pretty deep until I create a universal deb repository, which will come in the next few weeks):

http://software.virtualmin.com/debian/

Or:

http://software.virtualmin.com/ubuntu/

And wbm (Webmin package format):

http://software.virtualmin.com/wbm/

It's probably best to stick with what you started with. Though if you already have Virtualmin GPL installed, the upgrade to Professional should mostly leave your existing stack alone (but if you're particularly attached to the way its setup, you might just stick with installing via modules, anyway).

Note that you need to use your serial number and license key as username and password to login to any of the above download directories.

If you want to use yum, you can download the virtualmin-release file in the correct directory for your OS (rhel or fedora, and then following the trail down through version and arch).

We are nothing if not flexible.