At the moment, this is challenging. You would need to implement your own algorithm to detect the collision between the two arbitrary shapes, determine the normal vector, and the relative speed at the contact point if you want to model friction as well. I know of users who have created this algorithm in MATLAB and connected it to Simscape Multibody.
The best out-of-the-box solution we can offer through R2019a is the Simscape Multibody Contact Forces Library which only handles simple geometries (spheres, flat planes, cylinders, cones). If you can approximate your surface with those geometries, you may be able to do what you need.
If you can share the shapes you are working with, perhaps we can make some other recommendations. It will also help us ensure that we address this use case in a future release.