That model doesn't have a slot for a video card. From the manual--

PCI Express x1
Mini-Tower up to three full-height cards
Desktop up to three low-profile cards
Small Form Factor up to two low-profile cards
Ultra Small Form Factor none

Note that the above from the manual specifies "cards," not "GPU cards."  The MT, DT, and SFF can each take 1 GPU card.

MT's are the best for adding stuff.

Couldn't you, on the document for the Optiplex 990 USFF there is said to be a mini PCIe slot on the board, so if you connected a mini PCIe to PCIe x16, then wouldn't you be able to connect your graphics card????