Fujitsu Lifebook U749 secondary HDD

I own a Lifebook U749 and I would like to add a second hard drive, unfortunately I do no have a service manual and cannot check if this is feasable, I tried to put a Sabrent SSD PCIe NVME “M” key size 2242 in the slot that you can see in the attached pics (I think it’s the for the LTE module) but it does not fit into the socket even if the size and shape seems the correct ones, any idea if that slot is intended for secondary HDD or for other purposes?