Lucas' Radcore Cement Library Builder or Lucas' RCF Builder for short is a command line tool that allows you to build custom RCF files.
While not practical for modding the PC version of The Simpsons: Hit & Run thanks to the Custom Files hack, it has uses when modding the console versions of that game as well as The Simpsons: Road Rage.
This tool currently only supports version 1.2 of the RCF format, the version used by The Simpsons: Road Rage and The Simpsons: Hit & Run.
This tool is available from the Donut Team downloads page.
For tutorials on how to use the tool's command line arguments, see Tutorials.