I use the app AntiMicroX for controlling the mouse with a controller. I need to combine / and /home partitions as im running out of space on / but i cant use a regular mouse
I haven't done this before, but I think you can chroot into the mount directory, and run package manager commands in the mounted image to install another package.
Or I have an alternative suggestion that might or might not be easier. I've been hearing a lot about immutable/atomic distros, and people designing their own images. You could make your own ublue image, for example, with whatever you want on it.
A promising looking starting point is github:ublue-os/startingpoint. Ignore the "Installation" instructions, and follow the "ISO" instructions instead.
Or I saw recently an announcement of a new way to build atomic images that is supposed to be easier than ever, BlueBuild