usb – Can I flash a custom fastboot mode image on a Samsung device to boot custom devices without tripping Knox

I’m a noob and don’t even know if these are seperate images, but I do know that Samsung has no fastboot mode as it is stuck entering it and instead has a download mode.

The question is, can I still put something there to boot, like u-boot, will it trip knox and will it be able to boot from sd card or an usb stick.