You could add the Synocommunity as a package source, instructions here and then install Jellyfin from their repository in the Package Center. Jellyfin is available on their repository.

A disadvantage of this is that you're dependent on the maintainer of the package on the Synocommunity and it could take a while for new versions to be uploaded to their repository. If you use Docker, then you can basically get new versions immediately.

There are ways to get Docker running on an ARM-based Synology, it seems: https://stackoverflow.com/questions/52520008/can-i-install-docker-on-arm8-based-synology-nas

This is plain Docker, though. So you'll probably want to setup Portainer as well.

Does this help?

There you go: https://synocommunity.com/package/jellyfin

You need to install the SynoCommunity repository. Haven't tested it (I am still with Plex), so YMMV.