I thought that the community bulletin is for anouncements.
But I'm surprised to see this question: Ramadan and ramadhan tags should be merged
Anybody knows why? And who can add/modify to it?
For the most part, the Community Bulletin will populate itself with recent meta posts which are both well-voted and active enough to be deemed "important" (moderators can tag any meta question featured to prioritize it). If there was a recent blog post (from either the Stack Exchange blog, or the Islam.SE community blog) or a scheduled event, those will take precedence, but when possible the Bulletin will try to post up to four items so there will usually be at least one meta post on there if there's been any recent (and useful) meta activity.
The following discussion on MSO explains the selection algorithm in better detail (the post is over two years old, but as far as I know is still valid): How is the contents of the Community Bulletin determined?
Pretty much any user can participate in meta (the reputation bar is intentionally low), so while there's not really any way to control how "hot" a question gets (and thus whether it will show up in the bulletin) it is technically possible for any user's post to be included.