I used to comparing many experiment data (several years ago). At that time, I added some custom markers by modifying some source files. (Originally I started modifying from ROOT 5.)
Modified files are in the attached file, marker.tar.gz, which was based on root-6.09.02.
I confirmed that added markers were saved properly as ps, pdf, gif, pgn format in Linux and Mac, but I haven’t tested on other systems like Windows.
I don’t think most people need that many markers, but I hope it helps people who need more markers or like to modifying markers.