Yes, you can. And I just did mine and the speed (and size) of the fusion drive is awesome. Here are the brief steps:
# You will need the flex cable to connect the SSD to your mini motherboard. You can get it from Ebay or Taobao. Like this one (https://www.ebay.com/itm/PCI-E-flex-cable-from-2014-Apple-Mac-Mini-PCIE-NO-Flash-SSD-drive-SHIPS-FAST/283430264439?hash=item41fdc29e77:g:GCIAAOSw6XJbBflp&frcectupt=true) or choose one that has SSD come with it.
# Open the mac mini’s back using special TR-6 screw driver. Note, not regular T-6 screw driver. You can buy one from Amazon (https://www.amazon.com/gp/product/B01BG8P2Q6/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1)
# Install the SSD drive with flex cable, You will also need a regular screw to secure the SSD on the top side. I found one from another laptop’s internal wifi module.
# Boot up the mac and check the hardware, you will see two hard drives, one for the HDD and another for the SSD.
# Follow apple’s instructions to merge the two drives into a fusion drive. Note that you have to wipe the system so better backup. https://support.apple.com/en-us/HT207584
# For me, I put in a 256 SSD and get a total of 1.25TB fusion drive. Enjoy the speed (and capacity)!