Chris,
Yes, you can create a Simscape component which is just built from other Simscape components, this is called a composite component. Look at this example which creates a single Simscape block with the same behavior as the DC motor subsystem in the model ssc_dcmotor: https://www.mathworks.com/help/physmod/simscape/lang/composite-component-dc-motor.html
Of course you also could create a mask over a Simulink subsystem to get a similar "block".
