Actually to do this, the forums would all need to have the same keyword but each one would need a different Bo_Cat and Bo_CatName. This might be possible, but would take some trickery to achieve.
Basically, to test if this will work you can follow these steps:
You will need to create your main announcements forum just like normal. For this example I will assume you gave this forum the Keyword of "announcements". Next you will need to create another forum in a different category, give it a different Keyword but the same description as the first one. I will assume you gave this forum the keyword of "temporary".
After this you can go into the admin area, and into the database manager. From here you will want to do the following sql command:
UPDATE w3t_Boards SET Bo_Keyword='announcements' WHERE Bo_Keyword='temporary'
At this point you should have 2 duplicate forums in 2 different categories and whatever you post in one should appear in the other.
