Is SD UPDATER different than SD Updater? I was thinking you wanted to load additional apps to an SD card and launch them from the SD Updater menu. Is that correct?
Otherwise, if you want to flash other standalone apps (not launched from the SD Updater menu) to the M5Stack Fire and you are using Arduino, then no that is not possible (to my knowledge). Once you load a sketch to the device, writing/flashing another sketch will overwrite the first.
If using UIFlow, it has a built-in Apps menu. You can download many MicroPython programs to the device and select the one you want to run from the Apps menu.