Retroarch is already on Steam as well as other emulators.

https://store.steampowered.com/app/1118310/RetroArch/

Even if you didn't want to use the Steam versions. Steam OS is essentially a customised Arch Linux and you can install stuff as you would on other Linux distros e.g. via packages and flathub. Basically it is a regular computer underneath. That is why I am very excited about this Steam box.

https://www.gamingonlinux.com/guides/view/how-to-install-ext...

Isn't SteamOS immutable? Can you layer packages on it like you can with Fedora Silverblue?

You can add packages, but they can be wiped by the updates. Flatpaks work seamlessly and because of the Deck's popularity, most everything you would want is available in flatpak form

On the Steam Deck you boot into desktop mode and it’s a standard Linux. Install what you want. I have Heroic Launcher on mine, running games from GOG and Epic alongside Steam games.

I sure have installed a bunch of emulators on my Deck. It’s not too hard to get individual games to show up in the main Steam menu, iirc. Haven’t really fiddled with them since initial setup though.