Older CoCreate software (now known as PTC Creo Elements/Direct) did support both import and export of DXF format and thus should be your main choice for MI to DXF conversion. There is a special CAD translation program as part of CoCreate that can be used for this job. No 3rd party software for this conversion is known to us.
Convert CoCreate CAD drawing file to AutoCAD DXF Drawing Exchange Format.
We found one software available in the database for this conversion.
Microsoft Windows software - convert mi to dxf on Windows
An older 3D and 2D CAD design software
Convert from mi 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.