Easiest way to add another drive is to plug an external drive into the USB3 port on your laptop. USB3 is fast enough to give you good perforamnce. But this only works when your laptop is at your desk.
Next easiest is a guess. I've used several thinkpad laptops with two HDDs -- the second one fits in the optical drive bay. From your post I'm guessing that you can do the same thing with this alienware -- buy a caddy that holds a second HDD and replace optical drive. ".. I also got the BluRay player which I know sometimes takes up a HDD space..." For the thinkpad swapping out the second drive and replacing it with the optical took about 5 seconds and no tools. I found I never used the optical drive.
You can also network attach a disk drive (e.g. a NAS drive).