Well, you can try copying this file (see below) from one computer (the one online) to another (offline one), but i doubt it'll work. This is the virus database file alright, but avast! will have to refresh the data about new signature somehow. Have no clue how. Maybe log off and log on again or system restart. Haven't tested this. Oh and you'll have to disable Self-Defense to pull this one off.
C:\Program Files\Alwil Software\Avast4\DATA\400.vps