The Assembly object exposes several methods for working with its component Subassemblies. There are methods for adding, copying, inserting, mirroring, and replacing Subassemblies.
In this example, a stock subassembly is imported, then added to a drawing Assembly object, then mirrored:
// Add stock subassembly to drawing ObjectId subassemblyId = _civildoc.SubassemblyCollection.ImportStockSubassembly("Imported subassembly", "Subassembly.BasicLane", offsetLocation); // Add stock subassembly to assembly AssemblyGroup assemblyGroup = assembly.AddSubassembly(subassemblyId); // mirror subassembly AssemblyGroup mirrorAssemblyGroup = assembly.MirrorSubassembly(subassemblyId);