There is a simple command line tool which converts NRG Nero CD images into ISO images. The tool is called nrg2iso and it's included in the repositories, so to install it type as root:
Read it here
Another handy tool is iat, which allows to convert many image formats to ISO. According to the manual page, iat supports the following images: BIN, MDF, PDI, CDI, NRG, B5I. Although I only tested it on NRG, I'm sure it works for all the formats mentioned above.