Well I dont know, if this is relevant to your current problem but the .x to .ltb could be because you need the correct conversion process. Heres a list of .x extentions.
.x - Alysis SuperDisk self-extracting archive
.x - Microsoft Direct-X SDK
.x - Stardent AVS X bitmap image
.x - X-file animation file Direct3D
.x - FPS Creator Model file
.x - Chemical modeller output file
.x - XBLite program file
.x - Amapi 3D modeling file
.x - Xview object file
.x - XBLite source code file
.x - X11 server import image
.x - Human68k operating system executable file
Figure out which one your modding then look up the proper direct x source code conversion into lithtech.
Since there are no easy button tools to do it.
I'm sure that lithtech and direct x have alot of conversion source for this in thier sites.
Hey make a tool that does it and use it as a job application for one of the companies
Just a pinch of reverse engineering and wala.
With regards,
Debo