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.