It depends on the exact model XPS 15 you have, and then upon the type of battery the XPS 15 has. The newer XPS 15 models (9500 and 9510) will not take a 2.5" drive internally at all. The older XPS 9530, 9550, 9560, 9570, 7590) can take an internal 2.5" drive if you have a 3-cell battery installed.  They cannot if you have the larger 6-cell battery.  And even if you do have the 3-cell battery, you will need a mounting bracket, cable and screws that, unless the system shipped from Dell with a 2.5" drive, won't be present in the system.  You'll need to remove the base cover to check the situation before making plans to move the drive. You can also mount the drive in a 2.5" drive enclosure and plug it in by USB to access the data that way, in the event your specific system doesn't take a 2.5" drive internally.
Dear all,   I would like to enquire for how to successfully implant a secondary HDD inside my XPS 15 PC. I do recall that this model possesses a spare HDD slot, but do feel free to correct me. I would very much appreciate it if you could send me some simple and brief instructions or a tutorial on how to carry this out without any hardware or software errors - I wish to not experience any technical difficulties since I have a lot of data on this PC. When I say to implant this HDD, I intend to open up the necessary parts of the PC in order to access the respective HDD slot.    The community's assistance is always highly appreciated. Thank you.   With my kind regards, S_Mni2LliP_11 
Transferring External HDD Inside Dell XPS 15