I dimly recall trying to work with BGL format dictionaries before, and after doing some searching, I see it was five years ago based on the following post:
https://www.mobileread.com/forums/sho...&postcount=130
So, you have to convert BGL to .dict, then to .xdxf, and finally to the PocketBook format. The converter.exe program doesn't work with BGL files itself.