This program isn't really for specifically modding Cossacks or American Conquest but you can use it when you want to show your new unit animations to other people. Because most 3D unit programs like Poser, 3D Studio etc. will save an animation as an .avi file they are often very big files and also has to be played in a media player, if you have the animation in .gif format you can put it on any website and it will display straight from there. Movie to Animated GIF Converter will convert an .avi file to .gif format, its a great program that is fast and has several options.
The MD PiX tool will assign all the points/coordinates on a building where special effects appear such as fire, smoke, the path where a unit enters or exits a building, block points where the building is placed (so units can't walk straight through the building but have to walk around) and also where the peasants go when they construct the building.