So ... the 'hard way'! :\
An .mbz file is really a gunzip (a tar.gz) ... which isn't a pkzip/other zip.
Put the .mbz in a directory (like 'work') then un-compress from 'work'. Do you edits there as well. When done editing *be inside* 'work' to rebuild a .mbz. Reason, moodle restore process expects to find a moodle_backup.xml file at the root of what is uncompressed during the initial stage of a restore. That file is like a road map for the entire course and other .xml files/folders in it.
From 'work':
tar zcvf somebackup.mbz ./*
That should create a file called 'somebackup.mbz' in 'work'.
Am guessing that questions.xml is the only file needing to be edited ... tags in that file for the category of the bank should be top of the questions category for the course.
You might be doing multiple attempts at getting questions.xml right so that it does restore ... so put on a pot of coffee! :\
Good luck!
'SoS', Ken