[Question] How to modify ffu file?

Hi, I have Xiaomi Mi4 with unlocked bootlocker, Windows Phone Internals not working for this phone unfortunly, even not seeing phone and I have question: How can I push .bin partition files (or other filetypes, depending on tool that open ffu) into ffu file? I found many such questions, but always one person answering "you can't flash custom ROMs" or "you can't flash unsigned ROMs". I'm not best programmer on the earth, but I think with unlocked bootloader I can flash everything. Thank you in advance and English is not my native language so I apologise for possibly mistakes. :)