I am kinda new into scripting and was wondering how i can add a new button here:
I want to make my own winamp-controller, so i need a play, next, stop button and so on..

you can. using dll's, it's not a default option

Its not the easies tthing in the world. I tryed it, but I had to remake all the default buttong, and make my way down there. dll seem to mount the toolbar in the same spot, so u have to overlay yhre cuttent bar. >:\ (Thats why earlier I was looking for an alias that could be used just the same as the trigger of all those buttons) anyways, the ones you want are ktools and mdx if you chose to.