In order to simulate collision detection and reaction, you have to model both the resultant motion forces and the shapes of the colliding surfaces.
In SimMechanics you can model the center of mass of an object and the point where the force is applied. However, there is no way to define the exact shape of a body( ie: no surface modelling is possible).
Currently, the best available method is to calculate the reaction to particular known input forces (ie: implement the reaction forces).
For an example implementation on modelling reaction forces, review the Crash Test Dummy demo available with SimMechanics. The model makes use of the Virtual Reality Toolbox (VR scene). The model is named:
mech_airbag_vr
The attached AVI file below (crash_test_dummy.avi) illustrates the result of the mech_airbag_vr.mdl model simulation.
