As root, pico /etc/X11/XF86Config-4

Make sure this line already exists in Section "ServerLayout":
  InputDevice "Mouse0" "CorePointer"
And then add this line in Section "ServerLayout":
  InputDevice "USB Mouse" "SendCoreEvents"

Then define 2 mice - the pointer stick & the USB mouse.

Section "InputDevice"
  Identifier "Mouse0"
  Driver "mouse"
  Option "Device" "/dev/mouse"
  Option "Protocol" "PS/2"
  Option "Emulate3Buttons" "no"
  Option "ZAxisMapping" "4 5"
EndSection

Section "InputDevice"
  Identifier "USB Mouse"
  Driver "mouse"
  Option "Device" "/dev/input/mice"
  Option "Protocol" "ImPS/2"
  Option "Emulate3Buttons" "yes"
  Option "ZAxisMapping" "4 5"
  Option "Buttons" "5"
EndSection

You'll need to restart X Windows for your addition to work.

Note that sometimes I've needed to use /dev/psaux instead of /dev/mouse above in order to get the pointer to work.

Source: Linux on the Road: A Guide for Laptops and Mobile Devices: Chapter 4. Hardware In Detail: Pointing Devices - Mice and Their Relatives
http://www.mobilix.org/Mobile-Guide.db/mobile-guide-p2c1s7-pointing-devices.html