You can't POP them from any folders you have created on the server.
What you can *possibly* do and I mean *possibly* becuase I have not tried it but you could rename your Inbox on the server to Inbox.sav, then rename the folders you want to import one-by-one to Inbox and then POP it. Then either rename it back or delete it. Then rename the next one Inbox and POP it.
As I said I have not tried this but it sounds like it should work.
This is the main reason I POP my mail in the first place.
When you are done POPing the other folders rename your Inbox.sav back to Inbox.