which works fine for MP3 files, to send them to foobar's playlist. However, i would also like to add MP4 files, or even other music file formats. Is it possible?
That´s what i expected, too. But i have just changed a few things, registered MP4 to foobar (was "Movies" before), unregistered it from Light Alloy video player´s list, but still i get the properties dialog of that file popping up. Which is strange. I also removed it from Opus´ internal list, in "file type groups". It´s the same.
/ADD is not part of Opus, if that's what you're asking. It is part of Foobar2000's command line syntax.
Each tool that you can run via the command line may have a completely different syntax to any other tool, if it wants to. A command line is just a string of text passed to another program, and it is up to each tool to decide how it's command line should work.