i know it has been asked a few times, but still i didn't

i know it has been asked a few times, but still i didn't find the answer.
i have a mu site for my students, and i want to have the same catergories in all the sites i want to be able to add categories, after the indvidual blogs have been created. any advice?
i am using the threewp broadcast plug-in so to post between the sites, and i want to be sure every one uses the same categories