AFAIK, single session disk image in NRG format is a 600 bytes header and standard iso data after that. To remove the first 600 bytes, we can use dd utility from terminal.

The command format as follow,

dd if=source.nrg of=target.iso bs=512 skip=600

Replace source.nrg with your NRG file and target.iso with your desired target iso file name.

Good luck.