It can't resolve the name because it can't find a PushMessageFields
in the http://example.com/SomeConfigProperties
namespace. There is none. You can't redefine types from a different target namespace.
Unless you have other errors, it should work if you change the target namespace of your new stylesheet to match the target namespace of the imported one. In your second XSD replace:
<xs:schema xmlns="http://example.com/SomeConfigProperties"
targetNamespace="http://example.com/SomeConfigProperties" ... >
with
<xs:schema xmlns="http://example.com/configProperties"
targetNamespace="http://example.com/configProperties" ...>