The short(er) answer

As you may be aware, TurnKey is provided as a "monolithic" server build. As such, each server is designed to be a "stand alone" self contained server. Most users use TurnKey within a VM of some sort; whether running on a local instance of virtualbox (or similar) for development, local hypervisor (such as ProxmoxVE) for production or a VPS of some sort in the cloud (e.g. via the Hub).

Obviously that adds redundancy, but for many users, that's a plus. So on face value the answer is no, there is no way to just "drop in" an additional appliance into/onto an existing appliance.

The longer answer

Having said that though, TurnKey provides a range of different builds. The two that may be of particular interest to you in your scenario are the LXC and Docker builds. If you are unfamiliar with either of these "container" technologies, then I would suggest that the Docker build might be the best option. TurnKey LXC guests are streamlined for usage with our TurnKey LXC host appliance and/or Proxmox and will require a fair bit of work to replicate the functionality that we (or Proxmox) provide on your existing appliance. Docker on the other hand, should be much easier to set up and get going.

Assuming you want to go that way, I suggest that you do a full backup of your current server (i.e. create a "clone"/"snapshot" of the full root volume) before you start. Installation of Docker will make some fairly significant networking changes so if things go pear shaped, without a full "clone" there is no going back...

Once you have installed Docker the only other piece of the puzzle will be to allow access of your Docker TurnKey MediaWiki appliance to the broader network. The easiest is to use a separate port(s), e.g. ports 8080/8443. And link them via the Docker configuration (as per the documentation).

The neater way would be to configure an additional Apache virtual host on your current server to provide a reverse proxy for MediaWiki. Depending on which way you want to go (and what sort of DNS setup you have) you could do that either via a sub-domain (e.g. wiki.example.com), or a sub-directory (e.g. www.example.com/wiki). Either way, you'll also need to adjust your current Observium Apache config to support that sort of config.

I'm happy to provide further "best effort" assistance, but I'll need some more specifics on your existing server and infrastructure, plus some clairty on how you'd like to proceed give the above info.

I hope that helps... :)