You can, but unfortunately it's a bit arduous. See
here.
To give a little more context, an APK is just a zip file with a standard file/directory structure inside. If you rename it from koreader-android-x86-v2019.11.apk to koreader-android-x86-v2019.11.apk.zip you can open it however you like. Then you have to edit your Lua files in assets/module.
So far this was relatively simple, but then once you're done you have to sign the zip file or it won't install, and your own signature will be incompatible with our signature. This means you'll have to uninstall the program to install your mod, or to return to a regular release. (There's no way around this on Android.)
Btw, depending on the specific changes it might be worth proposing them?