XSD Problem: How to only restrict an element/attribute to be used under another schema

Question

I need to solve this and looks like I need help.

Here is the problem definition

We have an existing schema X [X is an industry standard schema] for which we are building some extensions in a new schema Y (with a different target namespace].

Now the problem is we want to restrict usage of the elements/attributes of Schema Y to be only as members of defined elements/types of X. [schema validation should fail in case of invalid usage].

How do we achieve this? What is the best way to do this?

RM