Both .qbmb and .qbb files are used by QuickBooks software for some of their backup files, with the difference that the first file is used on Mac version and the second on Windows.
Apparently, you can rename one to another when changing platforms. However, some users reported that they experienced fatal errors messages when they tried to import their Quickbooks Mac backups to Windows version. Your best option would be to to contact QuickBooks support and check possible solution for qbmb to qbb conversion.Open QBMB file Open QBB file
A business accounting software from Intuit
An OS X version of popular accounting software