Hi Shinya,
If you are trying to use the standard Asset and Assembly upload workflows to do the conversion, there is some documentation in VEG application help that explains how to do a manifest file upload using the hotfolders. Please have a look at sections "Manifest File Processing", "Asset Upload Sample DS.manifest file" and "Adding Hot Folders". This approach will store viewables and original files in the VEG database.
Creating a new workflow that is neither Asset or Assembly that opens up a CAD file and saves it to RH using that is triggered using a hotfolder is also doable but the explanation would be a bit more involved. If you are considering this option it is worth mentioning that in the VEG SP04 (currently scheduled to be available from 22nd of November this year) is adding a new workflow that exposes a simplified interface to the conversion process and does not store anything in VEG database. This new workflow can be invoked both through hotfolder and through a REST-full web api. Please let me know if you are interested and I can share some more details.
Kind Regards,
Marko