What you want to do is calculate the stiffness matrix before delamination and then after delamination. The before delamination stiffness matrix can be calculated in ACP (Pre). To calculate the after delamination stiffness matrix, first you need to do the delamination simulation in Mechanical (apply loads and BCs) and then use a sampling point in ACP (Post):
Let me know if this works for you. If it does not work, then I think you will have to calculate the stiffness matrix manually from the results (displacements) of different load cases in Mechanical.
Let me know how it goes.