Hi All - It’s unfortunately hard for us to answer these questions as we don’t have access to a current KEGG license, and the official KEGG installation has likely evolved since the time of HUMAnN 1.0 (which is when these files were first built).
If you’re doing custom alignment in HUMAnN (to KEGG or otherwise) the goal is to have 1) a file with all your sequences whose headers appear in 2) the id-mapping file (along with columns for functional category and taxonomy). Historically “1” was a file called genes.pep in the KEGG installation, and the id-mapping could be built from other KEGG-supplied files.