The rear of the G36 Receiver is closed whereas the SL9 Receiver's rear area is open to allow for the fixed stock to fit. You'll need the SL9 Receiver to use an SL9 Stock.
This diagram will show you an SL9 Conversion Kit, and should also show you how different the G36 and SL9 Receivers are, and why the SL9 Receiver is certainly necessary given the size and shape of the Lower Receiver/Stock assembly.