How to copy IMAP mailbox

[Gerardo Ballabio <gerardo ballabio gmail com>]

I have another question please.

I want to use Balsa to make a backup copy of my Gmail mailbox, that
is, copy the messages to a local mailbox, but still keep them on
Gmail. I can't find how to do that. It seems I can only move messages,
not copy them. There are in fact two Copy items in the menus (one
under Edit and one under Message) but there isn't any Paste and when I
click Copy, nothing seems to happen (how is that supposed to work?).

Maybe I'm missing something obvious...

Can you help me please?

Gerardo