Yes :)

In theory it should be as easy as copy and pasting, but I don’t know if that wouldn’t fail because of a timeout. Though, I have managed to wget a Google photos backup directly into JuiceFS and extract + process it there, just took like a day of time.

Mounting your Immich dirs there is all it should take. I would be careful with database directories/sqlite, they seem to bug out when mounted in JuiceFS