If you are really good, you can fuse the SSD and the HDD to form a Fusion Drive. There are several articles online, one particularly good one by MacWorld. This will combine the SSD and HDD into one logical volume. OS X is smart enough to put the OS and your most frequently used files on the SSD, giving you the speed, and putting the large files on the HDD. This gives you speed without sacrificing size.
I bought my 2015 4K 21.5” iMac with the256 GB PCIe SSD, and now I’m hoping to add a SATA drive to the vacant bay. What parts do I need to make this happen? Are any already there in the SSD-only versions?