It's very complex. Plus, umodel can handle a lot of different versions of the mesh formats (even UDK has a lot of format variations during its development).
Your question does not look like "help me", it's more close to "do this for me"
Again, if you want to make a CONVERTER from one package format to another (despite I've told you that this is impossible), you'll need to read a COMPLETE mesh (and a lot of other Unreal engine classes too), while I'm reading only the first half of data until acquire everything needed for mesh representation. I'm not handling collision, physics, data for shadow generation etc.