You may find helpful the same that was asked on SO a couple of times, for example here and here. Another free source might be the one described on Microsoft's Technet, as it relates to limitations present in MS SQL Server products (also showing a real life example related to your question), where <xsd:include/>s are not supported. If you don't have the ability to compile and run that code, you can still use it as a "script" with little help from a C# script engine such as this.
However, as I've pointed out here, the <xsd:import/>s cannot be refactored "out" given the limitation where a schema file can only target one namespace.
If you're ever into a really serious refactoring scenario such as the one described here, you may have to consider something else...
