Add 0 byte in linux elf file

I have a elf linux file that I need can I lost source code, I can not re create all I only want change a small thing, for that I need add a 0 byte in this file and with this byte I can change using asm to call a hello in screen, later I will change this hello code to other code that are with access for global variables and functions of the file, your job is only print the hello code and detail step by step to technicall speaking what was necessary to allow the elf file show the hello in console mode

I have three different files need to the same for three and documentation step by step too for three

Assembler Armado de muebles Ensamblador x86/x64

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