CA Question: Other custom unit formations?

There is one custom unit formation available that I am aware of: horde. This formation is defined by an image file (horde.tga.) Is it possible to create other formation names and use other simple images like the horde.tga. I tried creating a new formation with a new name back in 1.2, but it didn't work. I was able to alter horde.tga itself without difficulty and used the modified horde formation some as a test, but could not create new ones. (I don't have any specific formation in mind, just curious.) Are there any hidden formation name slots that could be used if someone created a new tga?