If any program is capable of conversion between .wpostx and .wpost files, it would be Microsoft Windows Live Writer program from which both of these files originate. It is unlikely, you can achieve wpostx to wpost conversion by other means using 3rd party software.
Convert Microsoft Windows Live Writer post to Microsoft Windows Live Writer post file.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert wpostx to wpost on Windows
Progam for making articles on popular blog services
Convert from wpostx 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.