Not that I know of in a default phpBB installation
It's probably possible with an SQL, or even with a php script, but it will take longer to write those than to subscribe to each forum 1 by 1.
Alternatively you could edit the database directly.
Going from memory it should be something like: (disclaimer: this is memory, and memory does fail me sometimes so check this first)
1 = user_id (that's your user_id, probably 2)
2 = forum_id
3 = read_status (0 is probably read, 1 would then be unread, but it could well be the opposite)
In your case you would use all the forum id's and set everything to unread to start with.
Again disclaimer: I haven't tried this, nor will I : my suggestion is to do it using the board features (subscribe) since editing in the database is not recommended at all.