Sometimes you have to write it/modify yourself. It's the same thing as using assimilate to create the car file for you, probably have more options when writing your own. Like creating batch jobs. The car file refered to is possibly the "master".car that points to all the anims found the .GLA (so that carcass can join all of them into the final .GLA used in the game)
You have to use it in a command prompt like so: carcass ravensofts.car
I'm assuming you add your .XSI anim to the list so it can be added into your new custom .GLA.