The only way to produce .mwd documents is to use Mariner Write word processor for OS X. There is really no alternative here, but the software supports all kinds of of other text and document files, so you may be for example able to transform plain .txt files and save them as .mwd documents.
Convert Simple text file to Mariner Write document file.
We have found one software suitable in our database for this conversion.
Apple macOS / Mac OS X software - convert txt to mwd on OS X
A word processing program for Mac
Convert from txt 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.