This "conversion" is actually more of a standard workflow in Microsoft Project, i.e. creating new template (.mpt) based on existing and opened plan (.mpp). You can do this only in Microsoft Project with the default File ► Save as.. function.
Convert Microsoft Project plan to Microsoft Project template.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert mpp to mpt on Windows
A project management software program developed and sold by Microsoft
Convert from mpp 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.