Thanks for the message.
I am afraid I am not aware of an easy solution to convert a .MDL into a .SBSAR. The two files aren't related and a .SBSAR is the conclusion of a substance graph.
It obviously depends on which subscription you have, but you could also download .SBSAR and even .SBS trough the Adobe Substance 3D Assets library.
Cyril Dellenbach (Micro) | QA Support Artist | Adobe