Using some of the dedicated voice recording and dictaphone program it should be possible to process or even convert various recording formats between one another, or export them to some common multimedia file. So perhaps dct (NCH software) to dau (Express Dictate) conversion is doable.
Convert NCH Software dictation codec file to Express Dictate dictate record.
We found one software available in the database for this conversion.
Microsoft Windows software - convert dct to dau on Windows
A voice recording program that works like a dictaphone
Convert from dct 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.