It really does not support addons, even tho I do believe there was something about addons in the future for the game on of the letters. Might be very wrong about that tho, so don't take it for granted.
I know there is one DPS meter that works with FFXIV and I'm sure you can just google it to find it, what I'm not sure about tho, is whether you can get banned for using it or not and whether it is safe or not, because it most definitely is a third party program. I would say don't use it, but that's just an opinion of mine.

Tho on a personal note, you should be able to tell how are you doing even without the exact numbers, even the mere flying text is good enough idnicator for that, unless you want to go "Aha, I did better damage than X and Y, noobs."