Afaik there is no way, mainly because there is a small data loss.
When a pristine wud is converted to wup format, some info is lost: padding, ciphering key, location table, etc.
No one has wrote a wup to wud format packer because the resulting file will never match the source 1:1. Maybe that file could indeed run in cemu, so a small repacking utility could be handy after all.