Can I assume that if I bought a Mac mini with the 256GB SSD I could add a hard drive and hard drive cable later? Would this function as a fusion drive even though the SSD is 256GB as opposed to the 128GB SSD in Apple's official fusion drive?

Is the PCIe connector present and usable on the mid-range Mac mini? I don't need the ~$300 fusion drive but would like the comfort of knowing that I can add an extra SSD in the future.