Using the eSword 9 converter it should be quite possible to convert eSword .bbl modules to .bblx files, provided the original files are not protected. Seems like the easiest way to achieve this conversion as you can download the converter for free from the author's website.
Convert e-Sword module file to e-Sword bible file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert bbl to bblx on Windows
A dedicated eSword converter
Convert from bbl 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.