This series of iMac's has two drive connections. The first is the SATA III (6.0 GHz) that supports a standard HD. The second is acustom PCIe SSD blade drive.
As the configuration you bought was a SSD model you do have the SATA drive location free to support a second drive.
So, yes you could put in another SSD drive but you may find the limits of space will still be present if you don't get a larger SSD. Sadly, the larger SSD drives are costly! A 1, 2 or 4 TB SSHD drive maybe a better way to go. It offers a deep SSD cache, yet it's a standard spinning HD for the deeper storage.
Bear in mind the newer 'Thin Series' iMac's are a bear to open and if you don't use the correct tools and take your time you can damage the display. Follow the IFIXIT guides to the letter otherwise you could have the added expense of a new display assembly!