A beginning:

query 1:

This would remove any of the forced watched forums. If you are gonna mass add users, be sure to clear any previous ones to prevent double entries. You could change query 2 to avoid adding double entries, but this works fine too.

The number 150 must be replaced with the forum id of the forum you want to force the watch too.

Code
DELETE FROM ubbt_WATCH_LISTS WHERE WATCH_ID =150 AND WATCH_TYPE = 'f'

query 2:

Add all users to watch forum 150 (replace with your forum of choice).

Code
INSERT INTO ubbt_WATCH_LISTS( USER_ID, WATCH_ID, WATCH_NOTIFY_IMMEDIATE, WATCH_TYPE ) 
SELECT USER_ID, 150, 1, 'f'
FROM ubbt_USERS

These queries would be great to run once in a while (together with the 2 from gizmo) if you want to keep forcing them.

If not, you need to run the suggested queries in this topic once, and add a little code to the new user stuff that would insert the watch into the watchlist.

[Linked Image from siemons.org]