You cannot directly convert .spb files to .sbu. You would have to recover the contacts saved in .spb file using Samsung Kies and then proceed to again backup your device (not only contacts) to pro a .sbu file. There doesn't seem to be any other way to do this conversion, although it is not conversion in proper sense.
Convert Samsung Kies exported phone contacts library to Samsung Kies backup.
We found 2 software records in the database related to .spb to .sbu file format conversion.
Microsoft Windows software - convert spb to sbu on Windows
A free software for managing of latest Samsung mobile devices
Convert from spb file format
|Yes, Samsung Kies supports spb file conversion as a source file type.||Yes||Yes||spb editor||Yes||No||No||No|
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned spb to sbu. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Apple macOS / Mac OS X software - convert spb to sbu on OS X
Management and sync utility for Samsung mobile devices