Anyone know of an RPF file creation library?

I'm tempted to have a go at a tool to automate the replace vehicles to addon vehicles but the stumbling blocks are the rpf files. I know very little about the format, so was wondering if anyone knew of an rpf creation library that could be used.

I don't need to read encrypted files... or any rpf files for that matter, I just need to be able to build them or write to existing ones. I could build dummy ones in OpenIV, use them as resource assets and then simply write data to copies of them.