origamiguy has created a GUI tool to make your game MOD-ready (requires .NET 4.0 framework).

UNGTC - ATD archive tool.
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This tool will extract and rearchive LEGO Racers 2 and Drome Racers archives (the Attention To Detail or ATD LEGO video games).
A while back, I started reverse engineering the LEGO Racers 2 and Drome Racers archive file formats (hence forth refered to as GTC). I figured out what almost everything was, but got stuck on the compression used in the GTC. In my extended absence from the site, and have been tinkering with the files, tagging up bytes in a hex editor, determining which bytes were litterals, which were backreferences, and which ones said what did what thorugh experimental byte injection. This would likely have never happened if not for this amazing find by Cyrem. Fast forward to September 1st, I finished writing code that would successfully decompress GAMEDATA.GTC. It didn't extract the files, and couldn't write its own COMPRESS.INF file, but the game would load and play, with some minor speed improvements. Fast forward again to today, and I have finished writing UNGTC, a tool that can both unpack the archives and create new ones!
UNGTC is writtin in pure C++, and is very fast. How fast it runs depends on the IO speeds of your drive. With a RAM disk, it can be as fast as a few seconds (5400 RPM drives are much slower). Full source code is included, as are binaries for Windows and Mac. If you are running Linux, compile the source yourself, there are too many processor architectures for me to provide binaries for all of them.
UNGTC is a command-line application, simple give it the folder containing GAMEDATA.GTC, FILELIST.INF, and COMPRESS.INF to extract, or the extracted folder to archive, as the first argument. On Windows, this is easily done by draggin-and-dropping the folder onto ungtc.exe.
Some technical notes: UNGTC does create slightly differently than the GTC's that come with the game, but they are 100% compatible, and more efficient. UNGTC does not compress the archives it creates in any way. Since you would not save mush space on modern drives and it would be a slow waste of processor power, I do not plan on adding this ability. UNGTC automatically detects the 2 different formats and uses a meta data file called .GTC to store what version for the file forwat was extracted, for use in rearchiving. It is a plain text file. The Windows executable it 32-bit. As it uses 32-bit opperations, it would not really benefit from being 64-bit. The Mac executable is 64-bit. Compiled using MinGW g++ (Win) and Xcode g++ (Mac). Other Notes
Unfortunately, I have tried everything I could think of to make it load the files from the drive instead of from the archive, monitoring the game with Process Monitor for any changes, but it seems to only work with archives.
To run the game without having to keep rebuilding the archive, check out this topic.
Or this tool by origamiguy (which will also extract the GAMEDATA files).
Many of the files are normal files and I expect that many other files have existing tools. Good luck in deciphering the files!