The 999 file extension is or was used by some Sage applications for their backup files. This type of backup was used on the old Sega games and there is no more information about this file.
How to open:
Use Sage programs to work with *.999 files.
How to convert:
As far as we know, this .999 file type can't be converted to any other file format. This is usually the case of system, configuration, temporary, or data files containing data exclusive to only one software and used for its own purposes. Also some proprietary or closed file formats cannot be converted to more common file types in order to protect the intellectual property of the developer, which is for example the case of some DRM-protected multimedia files.