How to load the sdf file of closed chain mechanism in Isaac sim?

Isaac sim can already support the dynamics simulation of closed-loop mechanisms, but it only supports the import of urdf files. The urdf file does not support describing robots with closed-chain structures. It needs to be described using sdf. So how to import the sdf robot description file into isaac sim? I know that isaac sim can support importing step files and then adding constraints through GUI interface, but the graphical operation is not precise enough, making it difficult to model a complex robot.

The picture shows the closed-chain legged robot that needs to be simulated.