Top-down Design

I am familiar with the Top-down Design approach but, have one question. How do you intergrate a known part/ sub-assembly into the design process?

Example:

Suppose I want to use a existing standard motor. I have a top level assembly with a skeleton model. The motor will eventually be assembled at a later time but, I need it to develope the skeleton surfaces that will be used to create the housing around the motor.  I need to pass information from the motor to the top-level skeleton. What are the best practices for this?

Thanks