I don't to add a second hard disk now but maybe in the future. Do you know if it's possible? (I mean there is a slot inside the laptop that allows me to do it? and the warranty stay ok?)
It is possible. And as long as you don't cause damage you won't invalidate the warranty.
I bought mine with just two m2's in RAID 0 and added an SSD and HDD I already had. The sheer number of disks it could take was one of the reasons I opted for it.
When you order a PC/laptop from PCS you must include at least one SSD/HDD drive or M.2 drive, PCS do not allow you to order without a drive for the Operating System (though you do not need to buy an OS from them - if you don't a trial/testing version will be on there)