I think what you want is retroarch
On the same sdcard? You could use PINN for multi boot and try to migrate the raspios partition there but tbh if you don't want to reinstall I thin it's easier and safer to keep each os install to a different sdcard. They are simply too cheap.
I was thinking like an app, the same way it would work on a regular desktop. Is that possible?
That’s called “RetroArch”.
Community about the single-board computers, micro-controllers and related projects.
Other RaspberryPi communities on Lemmy