Per Dell-Cares: Yes, you can. Here are the storage specs for this model at this link. The SSD type used is M.2 2280, so you can add another M.2 2280 for up to 4TB.
HelloMy laptop is a Dell Alienware x15 r2 and it comes with a 1TB hard disk. Can I add another 1TB SSD so I have 2 SSD?[IMAGE]  
x15 R2, add an additional SSD?