This conversion seems to not be possible. It appears that a .msj file is a temporary file generated when you work with graphics in Microsurvey CAD, so unsuitable for any conversion. It is likely that you convert .flx drawings only to other similar formats CAD formats via export function.
Convert Microsurvey CAD drawing file to MicroSurvey CAD temporary file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert flx to msj on Windows
A design software used by surveyors, contractors and engineers
Convert from flx 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.