3DXML format (Dassault Systems Universal 3D XML file) is a proprietary format generally only available to software produced by Dassault Systems, for example CATIA. As such, your best option for a .step (STEP product data) to 3DXML conversion would be to use CATIA, or any other similar software that has licensed the use of 3DXML format.
Convert ISO-10303 STEP product data to Dassault Systems Universal 3D XML document.
We have found 2 software records in our database eligible for .step to .3dxml file format conversion.
Microsoft Windows software - convert step to 3dxml on Windows
A PLM/CAD/CAM/CAE software suite
Convert from step 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 step 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