First of all, the NUC7i3BNB board is not sold as a bare board; it is only sold with enclosure (chassis).
If you purchase the NUC7i7BNH model, a bay for installing a 2.5" SATA SSD/HDD/SSHD is provided.
If you purchase the NUC7i7BNK model, however, no bay is provided (and indeed there is not enough room to accomodate one). If you would like to connect a SATA-based 2.5" or 3.5" drive, it is certainly possible, but you would need to do so with the NUC's cover removed. The specs for the special power connector are provided in the Technical Product Specification (TPS) for the board. It may be possible to purchase such a cable. Intel Customer Support will need to confirm if one is available and, if so, can point you where ut can be found on their spare parts site . Otherwise, check with vendors like MicroSATACables to see if they can provide one.
Hope this helps,