I want to create a polymer layer with large vacuum at the upper and lower region and put some water molecules at the lower region to let them diffuse through polymer layer into upper region. As the water molecules at the lower region can move across the Periodic Boundary, I plan to create a dummy atom layer at the lower part to prevent such movement. 

Questions:
1. Is it possible to create dummy atom in Materials Studio?
2. I try to use a graphene layer as alternative option now and freeze their coordinate, but the polymer (PE layer) in middle region will experience drifting and attracted to the graphene layer under NVT ensemble... What's the best way to avoid this?