Okay, The *.nrg file is a data disc, so I suppose it could be done, and I got nrg2iso about an hour after writing the post.
But I'm still wondering, if cdemu can mount *.nrg and *.bin and *.cue to a "virtual CD drive", then how can I really mount it? what are their filesystems to be written on mount -t filesystem /path/to/cdemu /mount/point ? or do they have
Thanks for replying anyway!