If you just want to add a quick image as a reference, but don’t need to cut any holes in it, you can use the Create>Textured Plane feature in Synchro (accessible by right clicking in the 3D window)
See this video:
If you need to create holes for excavations etc, we recommend adding the image as a texture to a block in either AutoCAD or SketchUp and add cuts for the excavated portions that can be removed as separate objects in Synchro at the right time.