mp4 to dmx conversion is probably searched in context of Valve's Source Filmmaker, with the .dmx files replacing the previously used .smd files. DMX is 3D ASCII format for storing 3D content.
It is unlikely any other tool can help you out with this, except Source Filmmaker. You can download it on Steam for free and and you find many guides and tutorials on the Internet (for example Valve Developer Community) that might help you out with your export.
Updated: January 17, 2024