When you backup your databases (.pdm) in PowerDesigner, one or more backup files will be generated which perhaps also include .bpb files. This is the only viable pdm to bpb conversion we can think of. If you are looking for something else, please write in the discussion below.
Convert PowerDesigner database to PowerDesigner business process backup file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert pdm to bpb on Windows
A business process and data modeling software
Convert from pdm 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.