For using USB-controllers on 9-pin (DB9) systems, except MegaDrive, you can also try these here.
"RYS MK-II" joystick/mouse-adapter:
Then there is the "TOM" joystick/mouse-adapter, which was mentioned before in the thread:
And there are also some other good solutions for using MegaDrive, SNES, NES, Playstation or analog PC-controllers (with DB15 connectors) on DB9-systems. Either from "Raphnet" or one of the five "64JPX" adapter-models or from Kipper2K shop the PSX-adapter (with Paul Willingham firmware) and so on.
Should work on all 9Pin systems, only on Megadrive i would not use any of these adapters, because the MD has little differeneces in the pin-outputs. And if the second Atari-7800 button will work on those adapters i am also not sure.
For example, the Raphnet SNES-to-Commodore/Atari adapter supports a second firebutton for Amiga-games. Anybody knows, if the second firebutton on the VCS-7800 is on the same pins like the second Amiga-firebutton? Is it the same?