You can run xf86config or xorgconf (depending on the xserver you aure using xorg or xf86) (I am not exactly sure of the names conf or config, XF or xf) and choose an usb mouse.
The changes will be written in /etc/X11/xf86config or xorg.conf (see also xf86onfig-4). This means 2 things.
1. You can edit the file manually to make the changes you need
2. You should make a copy of the configuration file because running the tool mentionned above can modify many things about the gui (screen resolution, mouse, keyboard, graphics card support...)