Yes, the HUD can use multiple DBs.
What you suggest is pretty much what is recommended for large DBs and datamining.
You could set up one database for your played hands, and then import observed hands into a separate database which you change every month (or whatever). See the option on the auto-import folder for which DB to import played/observed hands into.
As for backing up databases and moving them to remote storage - backup options will be added to PT3 at some point, but for now you can do it from Postgres itself, or from the command line. There are a couple of links in the
Database / Postgres FAQ.
Once the hand histories have been imported and you stop auto import, they are moved to a 'processed' folder (again, see the setting on the auto import tab). From here, you can zip/compress them and move them off to remote storage.