What you call DB9 is probably a serial port (or COM port like PC people call it). The Amiga's mouse port is an Atari-like game port, not a serial port. Serial devices do not work on the Amiga's game ports, they can even get damaged or damage the computer. It's a miracle that the trackball could work on it.
To get an USB device work on the Amiga's game port, you need one which can also "talk" PS/2 and a (rather expensive) PS/2-to-Amiga converter like the Cocolino or Topolino3 or Micromys.
