So my problem is that I have some pre-made playlist and when i open any media file this playlist is overwrite with new one that contain only this file. It is possible to add file that I opened at the end of current playlist?
I know of two options.
Drag the file into the playlist. It will then appear in it without replacing the playlist.
Disable the option that restricts MPC to a single instance. Files you click on will then appear in a new instance of MPC with its own playlist box.
Perhaps there is a way to do what you ask but I do not know it.