Hmm, how to respond to this...
First off, can you specify a bit more why and how you got into this problem? From what I read, I'm assuming that you've modded the .JAM archive contents a little or majorly, and would like to rebuild the modded .JAM, is that correct? If so, then paste the modded .JAM archive onto the JAMExtractor.exe, and it will build a new LEGO.JAM. To play with the modified LEGO.JAM, place the file in the archive in which your LEGORacers.exe is stored as well (often found in Program Files --> LEGO Media --> LEGO Racers). There should already be a LEGO.JAM in this archive, this probably is the original or former .JAM file. Opt for overwrite when you replace the 'original' LEGO.JAM with the modified LEGO.JAM, but also remember to have made a back-up of this original .JAM file, just in case.