If any programs can handle .topx and .bblx files and convert them between each other, it would be e-Sword or its utilities from which both of these file types originate. You may also find some stand-alone conversion program that should support topx to bblx conversion just fine.
Convert e-Sword topic note to e-Sword bible file.
We have found 2 software records in our database eligible for .topx to .bblx file format conversion.
Microsoft Windows software - convert topx to bblx on Windows
A bible study computer software
Convert from topx file format
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 topx to bblx. 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.
A dedicated eSword converter