If you are expecting the 1.9.x backups with users to come in cleanly to your 3.4.x, not sure that's possible.
Every 1.9.x course restored will not include users.
The files in the restored 3.4 course will be under 'Legacy' and not the 'typical' file system and once in Legacy it will be in Legacy for ever.
Plus, if there were any addons (mods/blocks) in the 1.9 courses that do NOT have a compatible version for 3.4, those also won't restore ... matter of fact, those might actually stop a restore dead cold in it's tracks.
OR are you asking how to program it so teachers can restore their 1.9.x course backups into the 3.4.x site? without such issues as above?
Getting 1.9.x backups to an area that 3.4 site see isn't a problem ...
in the 3.4.x moodle, setup a file system repository. Then, In the data directory for 3.4.x, cd moodle34data/repository ... at this point you have options ...
you could create a symlink per teacher's 1.9.x backup directory from the 3.4.x repository
or just one for all teachers to navigate.
OR you could copy all the moodle19data/##/backup/backup.zip files (where ## is the course ID for the teachers courses) to the 3.4's moodle34data/repository/19courses/ directory.
Use one course in the 3.4 called restores ... all teachers have access to it. New Categories have to exist in the 3.4 already, but teachers could use the 'restores' course, restore, navigate to their 1.9 backup zip, and attempt to restore to their category a new course.
Just remember, after doing all that (which ever method chosen), you, the admin, will have to deal with things like broken links etc. and all the issues such a project would bring over/across, etc. from that time forward.
You might want to contact the user at this posting:
and ask how it's going with Moodle Legacy files in a site that was migrated from a 1.9 many many years ago ... course backups and restores have issues now related to Legacy files.
To be honest ... think it best to start with a fresh 3.5.x and have teachers rebuild courses ... they will have to learn the new anyway.
2 cent opinion from one who has migrated 1.9 sites, used 1.9 backups to
restore to 2.x's, and tried other things .... like a 'command line poof'
to rid 2.x/3.x sites from just Legacy files.
'spirit of sharing', Ken