I have a more XSD files which contain the description of the message structure. Our application can use only one XSD so at the moment I need to merge the XSDs into one XSD manu
The function you're describing goes by the name of flattening and is available in commercial XML/XSD editors such as XMLSpy, oXygen XML Editor, and QTAssistant.
Be careful what you want. In my case I had a schema which was importing two other schema files. As all of them were using different namespaces, so they could not be "flatten" by XMLSpy. I guess this function is meant for something else then. Made the tool useless for me and I merged them manually again by copy&paste the import part into the base file.