Hello, I just spent like 5 minutes trying to figure this out and figured this might be a common question, so just posting the answer here for posterity. If PieFed hasn’t seen the community before you can add it by navigating to Topics -> All Communities -> Add Remote Community.
It seems like unseen communities don’t automatically get added when you try to migrate your user JSON, so be careful to add any remote communities that are missing.

Is there howto for that?
Edit - answering my own question. On Lemmy go to user->Settings->Import/Export Settings, on Piefed to Settings -> Import&Export.
The export was almost instantaneous, the import ran for a few minutes and crashed Piefed. I’ll try one more time and maybe manually edit the JSON to make it smaller if it crashes again.