I don’t see openhab doing that by itself so you have to use some other mechanisms.
Heres a quite new topic about backup scripts Backup/Restore scripts for Linux and MacOS. You could make a cron job that runs these scripts and uploads .zip files to another server. Is your nas only online at daytime or could you schedule it to power up during the night and let it run for an hour? Then maybe a python script could ping your nas to be sure it’s online and then upload the backups.
I’m not much of a coder myself so i can’t serve you the final solution but throwing some ideas.