Right now there's no way to do that... but I've just added support to ++ for the writetofile and addtofile commands. The below will work for next release:
"addtofile;c:\test.txt;=AUDIO||Audio Player" (Saves: AUDIO||Audio Player to file)
If you're actually trying to add multiple labels/text you will do:
"setvarbycode;part1;TRACKNAME||setvarbycode;part2; TAGARTIST||addtofile;c:\test.txt;=$part1$++$part2$ "
Remember that those commands work with a label code, so if you have a specific text, you should put a = in front of it so RR know it's just text.