I read the readme for that, and I have a question. Don't other NES translation IPS' expand their ROMs without needing a separate tool?
Clarifying what Pennywise said, almost all NES mappers require the last bank of PRG-ROM (usually the last 16KB) to remain at the end of PRG-ROM, so extra ROM space must be added before that. (so thus the last bank of PRG-ROM, as well as the entirety of CHR-ROM, if present, is relocated in the ROM file)
(because the 6502 expects to find vectors (pointers) to the interrupt routine, as well as the reset/start point of the code at the end of the 64KB address space, so most mapper designers found it easiest to permanently map a specific ROM bank to that point)