Transfer a Custom-Rigg with IK-Controllers to Blender

Hi, is there anybody who knows a workflow to transfer a custom rigg from 3ds max 18 to Blender 2.82+? The Rigg is used to work with MOCAP-fbx files and has additional IK-Controllers which I would like to be preserved. The goal is to transfer the Rigg with no or only minor adaptations. So far the import in Blender looks like in the image below.