You would need to add nodes to the graph and expose the controls of those nodes in order to add parameters to the material.
Just as an example, after using Send to Substance 3D Designer in Sampler, open the graph in Designer and add a blend node just ahead of the roughness output node. Set it to multiply. Feed that with a uniform Color node set to grayscale and Expose the gray value as a graph parameter called Roughness. That will allow you to adjust the Roughness from it's current value to completely smooth.