Adding HDD to SSD only XPS 9560?

Thank you, in that case I will consider just upgrading the SSD, but would i have to install things again such as drivers? Also what kind of 1TB SSDs could I use, can it be an SSHD? (https://www.amazon.co.uk/Seagate-ST1000LX001-Internal/dp/B00B99JUBQ/ref=sr_1_3?s=computers&ie=UTF8&qid=1536263166&sr=1-3&keywords=ssd%2Bfor%2Blaptop&refinements=p_n_size_browse-bin%3A10625607031&th=1)

Ask the seller which battery is in the system -- in most cases, configurations other than those that ship with a 2.5" drive have the 97WHr battery.  And while you can swap the battery, that will cut your battery runtime in half -- it may be OK for an FHD system, but don't even consider buying the 4K screen and using it with the 56 WHr battery.  You'll likely see only 3-4 hours of battery run if you do.

I see...I'm buying from a website that doesn't specify the battery size. Is there any way to find out from the other specs? And if i happen to have the larger battery, would it be possible to swap it out for the smaller one? Can I even buy the smaller one standalone? Thank you once again

Leave the OS on the existing SSD.  Moving it to a SATA SSD will be about half the speed - and moving it to an SSHD will be immensely slower.

> Also what kind of 1TB SSDs could I use, can it be an SSHD?

Nope, you need a m.2 2280 size SSD, preferably NVMe, I'd suggest the Samsung 970 evo 1TB. 

I assume a SATA type drive would work too, but much slower.