I've got an information from official cegui forum that only way how to do it, is to create custom resource provider by extending ResourceProvider
class and implement loadRawDataContainer
function. Then you have to initialize cegui system manually instead of calling bootstrapSystem
function.
Manual initialization:
guiRenderer = &Direct3D11Renderer::create(device, context);
if (System::getSingletonPtr()) CEGUI_THROW(InvalidRequestException("CEGUI::System object is already initialised."));
System::create(*guiRenderer, &ceguiResourceProvider); // inject custom resource provider