And ofcourse all the previous answers have been wrong.
As he is asking howto enable lock code on the device for mfe.

Bootloader cannot ever be changed, its readonly.

1. For getting mfe to work you either need to do full flash of the firmware. Tutorials are avail in the forum. (Its not enough to flash the kernel as that doesn't swich open mode off)

2. Second option is to find custom openmode pached devicelock debian package which enables device lock in open mode.

So for your question you want second option (as you didn't want to delete anything). But the downside is that I only know 3 persons in the world who has that deb package. So it might be a bit hard to find... so you might end up just reflashing your device.