Both .nba and .nbi files are backups produced by Nero BackItUp software. Whilst probably not possible to convert one to another, you could perhaps recover the contents somehow and produce a new backup. Anyways, only the original software will be of help here and no 3rd party products can help you.
Convert Nero BackItUp archive to Nero BackItUp backup information.
We found one software available in the database for this conversion.
Microsoft Windows software - convert nba to nbi on Windows
As backup utility used to backup selected data, or operating system as well as hard drive
Convert from nba 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.