Add a song into a playlist while listening to it? Is there a button to add the song that is being currently played into a playlist on my iPhone?
So i don't have to search it back and add it manually?
No - the playlists only get curated from the playlist tab.

The iPods with physical buttons have an "add to playlist" functionality that worked this way but there's nothing yet in iOS 4 or less to do a similar thing. The drawback there is everything got added to the "on the go" playlist - not an arbitrary one.