How can I add new folders,?

I want to put old emails in ahold folder to archive them before switching to IMAP.

I also can’t seem to download all recent email or else something is filtering what’s in my in basket.

If you mean exporting to the sd card, K-9 does not support that. If you mean a folder on the server, the POP3 protocol does not support that. The only option seems to be to forward important emails somewhere