It looks like obj to 3dm conversion is some sort of 3D object conversion and as such it can only be done using some 3D modeling tools or dedicated conversion utilities with support for both of these formats. Several 3D modeling tools can be used for this and you can even find some dedication, web-based online converters.
Open OBJ file Open 3DM fileA CAD 3D geometry translation software
A converter for 3D formats
3D CAD/CAM tool for product development
An online CAD conversion service
A 3D CAD translation software