I'm two years late to the party; however, I have a theory that may work.
External graphics card adapter kits have been on the market for use with laptops/notebooks.
Dell OptiPlex Ultra Small Form Factor GX620, 745, 755, 760, and SX280 do not have the appropriate connector ports for direct connection. They do not have any PCI-E lanes, mini-PCI-E, ExpressCard, or NGFF M.2, but they do have USB and USB 2.0 support.
Using a USB 2.0 to ExpressCard adapter from the computer to an ExpressCard to PCI-E graphics card adapter should work... IF the computer supports USB to PCI conversion (I don't know if it does). Of course, you will need a power source also.
Product information:
https://www.amazon.com/dp/B001KUOW9O/_encoding=UTF8?coliid=I32OH8RLXDHR0Z&colid=17TI8I5CLEYBK&psc=0#Ask
https://www.amazon.com/dp/B0725B6L99/_encoding=UTF8?coliid=I3I8FHM1OSO0HQ&colid=17TI8I5CLEYBK&psc=0
http://www.siig.com/it-products/usb/converters/usb-to-expresscard.html
https://item.taobao.com/item.htm?spm=a312a.7700824.w4004-16541360067.4.415ecc67Ud9pNv&id=37090263024
Here is a good video of the graphics card adapter:
https://www.youtube.com/watch?v=Dn71tIb25e0
Here is a good video that explains external card risers:
https://www.youtube.com/watch?v=en_JZSXqu0o
(He states in the comments : "Please Note:- You cannot connect the GPU Riser to your USB port. The usb cable is just used as an alternative to a ribbon cable, there is no usb interface. You cannot run the gpu riser from a usb port!")
Hopefully these ideas help with such a common question. I have not done this personally, (and haven't found anyone on the internet who has), and cannot confirm if it will work.
** It would cost the same just to custom install an aftermarket mini-itx into the case instead **