This will allow basic updates to an sfs using medium compression (-b 512K -comp xz -Xdict-size 50%). Use at own risk.
First download .pet below & install by clicking on .pet in filemanager. Then:
Place sfs to edit in an empty folder & open terminal.
cd into that empty folder & run updatesfse
make edits in utmp subfolder
when done, click OK
when compression done (could take some time), mynew.sfs appears in folder.