How can I modify Boot Camp Assistant so it can use bootable USB?
Boot Camp Assistant.app - among other apps in the Utilities folder - is protected by System Integrity Protection as outlined here.

By disabling SIP you will be able to edit the info.plist again.

So boot to Recovery Mode by pressing cmdR while booting.


Open Terminal.app (menubar -> Utilites -> Terminal)
Enter csrutil disable
Reboot your Mac 
Edit the file /Applications/Utilities/Boot Camp Assistant.app/Contents/Info.plist
Reboot again to Recovery Mode and enable SIP again: csrutil enable
Reboot your Mac




You should also be able to edit the file directly after booting to Recovery Mode without disabling SIP:

/Volumes/name_of_main_volume/usr/bin/nano /Volumes/name_of_main_volume/Applications/Utilities/Boot\ Camp\ Assistant.app/Contents/Info.plist


You'd have to make notes how and what to add though because you may have no real internet access.



Even if you can create a USB Windows installer, this does not necessarily mean that you can boot your Mac to Windows using this flash drive.