I've not had to get one myself, for a long time, but there's definitely external DVD drives
that you can get (from any supplier, that's just my own first port of call) that will work with practically any machine with USB capabilities (and, let's face it, it's a rare machine that hasn't), or I imagine there might be eSATA versions (might have better, or at least less bottlenecked, performance), if there's a suitable port. Search and ye shall find?
Hard to say if you can
fit an internal optical drive, without having a look at it myself. Is it a fancy fascia that covers up all drive bays, or a super-compact casing that is just chock full with the necessary high-performance components? But if you're unsure about it then getting someone who would
be sure to take a shufti might help confirm the situation, and advise you better.
(It has been a while since I made a PC specifically for high-performance gaming, and that one had a floppy
-drive slot, something that has been since discontinued as standard equipment for... reasons. (Air-flow and dust ingress problems. As if the floppy-slot, backed by the drive, was really any worse than all the other necessary cracks in the case. Conceivably optical drives could have been nixed for similar reasons, once install-from-download became a thing, etc.)
(Ninjaed. But, as I waffle on about other things, posting anyway.)