public class Main {

	public static void main(String[] args) throws IOException {

		// register the TVLPackage to ecore
		EPackage.Registry.INSTANCE.put(TVLPackage.eNS_URI, TVLPackage.eINSTANCE);
		Resource.Factory.Registry.INSTANCE.getExtensionToFactoryMap().put("tvl", new XMIResourceFactoryImpl());
		// register the resourceserviceprovider
		new TVLSupport().registerServices(true);
		// register the dsl
		Injector injector = new FCSSStandaloneSetup().createInjectorAndDoEMFRegistration();
		// load all model file to the resourceset
		XtextResourceSet resourceSet = injector.getInstance(XtextResourceSet.class);
		resourceSet.addLoadOption(XtextResource.OPTION_RESOLVE_ALL, Boolean.TRUE);
		Resource fcssResource = resourceSet.getResource(URI.createURI("/Users/Quentin/runtime-New_configuration/Test/Tune.fcss"), true);
		Resource tvlResource = resourceSet.getResource(URI.createURI("/Users/Quentin/runtime-New_configuration/Test/Tune.tvl"), true);
		// work with the resources (the elements come from Tune.tvl)
		fcssResource.load(null);
		Resource xmiResource = resourceSet.createResource(URI.createURI("/Users/Quentin/runtime-New_configuration/Test/Tune.xmi"));
		xmiResource.getContents().add(fcssResource.getContents().get(0));
		try {
			xmiResource.save(null);
		} catch (IOException e) {
			e.printStackTrace();
		}

	}
}

Exception in thread "main" org.eclipse.emf.ecore.resource.impl.ResourceSetImpl$1DiagnosticWrappedException: org.xml.sax.SAXParseException: Content is not allowed in prolog.

Resource tvlResource = resourceSet.getResource(URI.createURI("/Users/Quentin/runtime-New_configuration/Test/Tune.tvl"), true);

import org.eclipse.xtext.resource.generic.AbstractGenericResourceSupport;

import com.google.inject.Module;

public class TVLSupport extends AbstractGenericResourceSupport {

	@Override
	protected Module createGuiceModule() {
		return new TVLRuntimeModule();
	}

}