Sure! I make Signal backups on my Android device, sync them to my home server via FolderSync, and then run a nightly script that uses signalbackup-tools[1] to extract media from my family group chats and upload them to my immich server via their CLI.

[1] https://github.com/bepaald/signalbackup-tools

Couldn't you simply select the Signal folder that contains the media and let Immich monitor changes