The most obvious choice for CATPART to 3DXML conversion would be to use CATIA which not only produces the .catpart files, but also features support for 3DXML format. You may probably find other 3rd party CAD programs that can be used in the same manner, but all of them will be commercial products.
Convert CATIA part data to Dassault Systems Universal 3D XML document.
We found 2 software records in the database related to .catpart to .3dxml file format conversion.
Microsoft Windows software - convert catpart to 3dxml on Windows
A PLM/CAD/CAM/CAE software suite
Convert from catpart file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned catpart to 3dxml. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
A 3D product data optimalization software