How to add a new field to a post?

[Meorazhar]

I have found an old thread about adding custom fields to threads, and, although it was somehow helpful, i still can't figure out how to do this for posts. I need this new field to be visible and editable in all possible places (i.e. starting a new thread, making a new post in advanced mode and editing a post). Also it should be a dropdown (the values should be initiated through a separate plugin)

All further help would be appreciated.