MacBook Pro 2013 hard drive upgradable?
No, it cannot. Actually, there is no there is no drive-bay at all (standard or optical). The traditional, 2.5'' 9.5mm internal notebook drives (whether SSD or HDD) that you're thinking of are not compatible (internally) with this model.

This is what the solid state "drive" looks like inside late 2013 models:



As you can see from the iFixit photo above, the SSD is really just a stick of flash memory, connected via the PCIe bus. It's made up of 8 identical NAND flash modules (in densities of either 32, 64, or 128 GB). The 256 GB drive is pictured here. There are 8 32GB chips in total, 4 on each side. The chip density corresponds to the listed drive capacity, so 8x64GB Modules = 512 GB, etc.

Though it is possible to replace the SSD with a larger capacity one, there's currently no aftermarket upgrade available yet: 


  Unfortunately, the proprietary PCIe 2.0-based SSD in the "Late 2013" models is limited to a smaller "blade" option, but upgrade options no doubt are forthcoming, nevertheless.


Soon forthcoming, indeed.

Consider purchasing an external storage device, or exploring 'cloud' based storage options.