It appears that .nco files produced by Nero BackItUp are just ZIP archives with unique extension and such there is really no need to convert them to .zip file. Should you wish it, you can rename the extension from nco to zip and extract the content. Some file archivers may even recognize .nco files and you do not need to rename the suffix.
Convert Nero BackItUp compressed backup to WinZIP Compressed ZIP archive.
We found one software available in the database for this conversion.
Linux/Unix software - convert nco to zip on Linux
A Linux application used as frontend for popular compression tools
Convert from nco file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.