If I understand your question correctly, you have a robot that is supposed to pick up an object, so you are trying to add that object's mass to the end of the robot arm once it has reached that location. It sounds like you are using the PS Switch block to control which Frame is attached to the end of the robot. That method will not work -- the Physical Signals library is not intended to connect/disconnect Simscape Multibody Frames.
There are a number of ways to implement this. A common method is to put a stiff spring-damper between the Robot and the object where the spring stiffness and damping coefficient are dependent upon an Input Signal. You could use a Switch block to Switch between "connected" (coefficients >0) or "disconnected" (coefficients = 0). You could also use an External Force block which applies Forces and torques based on signals -- you could set those Input Signals to 0 when no object is in the robot's Hand or to nonzero values when the object is there. You will Need to very carefully define the value and orientation of the vector.
Some other items that might be useful:
Here is a pick-and-place model implemented in Simscape (not Simscape Multibody) http://www.mathworks.com/matlabcentral/fileexchange/38515-smart-manufacturing-robotics-cyber-physical-system
This contact forces library would be an option for having your Robot actually pick up the object
Hope this helps! --Steve
