Many 3D modeling or conversion tools nowadays should be capable of conversion of 3D models saved as .obj files to FBX format used by Autodesk Kaydara, which is essentially what obj to fbx conversion represents. This should be pretty straightforward conversion as OBJ is very common format and supported in all kinds of 3D modeling tools.
Open OBJ file Open FBX fileA web-based freemium 3D computer graphics software
A Mac version of the popular modeling tool
An FBX convertor utility from Autodesk
A 3D CAD translation software