Pmx Editor modeling tool for MikuMikuDance models (.pmx) also supports 3D graphics saved in Microsoft Direct-X SDK format (.x). As such it should be quite possible to import .x files to the program, process it and export it as .pmx file.
Convert Microsoft Direct-X SDK 3D graphics to MikuMikuDance model file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert x to pmx on Windows
PMD, PMX, X MikuMikuDance model editor
Convert from x 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.