Is there another app that could feed the data to citrine already now?

If you can get a jsonl file from a relay you can import it. Nostrudel can use citrine as a cache relay and it sends everything it fetches to citrine. I made i pull request for nostrsync a while back to add support for exporting your data as jsonl but i think the owner is busy and didnt review it yet. You can get a jsonl file using nak in your computer for now