If it is games you’re running with another OS, you must keep a part of your 2nd storage and change sudo lvcreate -T -n poolhd0 -l +100%FREE qubes to sudo lvcreate -T -n poolhd0 -l +50%FREE qubes or another size to keep space on the storage for installing another OS and then, install your games.
but I’m not competent enough to help you with that.
the other way would be to create an HVM and install an OS but as Qubes doesn’t manage graphics acceleration, to play… You’d have to do a passtrough and it’s really not easy.
Personnaly, i installed debian on a 1To to an external ssd and i boot on it when i have to use on “classic OS”. This is the simplest solution.