If memory serves you can use an art program and try up some markings. Then save it as a jpg(?) And then giving the markings a name. Once you're done, save the markings in the markings.cfg file.
I know I made an error somewhere because I have only tried it twice and that was a couple of years ago.
If you're talking about making a custom marking where you MAKE it yourself, then yes. You might need to make it compatible. But if you're talking about ones where you downloaded someplace then you would put them in like you have in the old one. Media>>Textures and then replace the .cfg file.