The file that you can edit to manually set your bindings is Controls.con found in your BF2 Profile directory ( "My Documents\Battlefield 2\Profiles\0001" is where mine is stored).

In there if you do a find for "wheel" you'll find these two default entries for your Mouse wheel.

Code:

ControlMap.addAxisToTriggerMapping c_GIMouseWheelUp -1 IDFMouse IDAxis_2 0
ControlMap.addAxisToTriggerMapping -1 c_GIMouseWheelDown IDFMouse IDAxis_2 0

Using this as a search, I found this thread that can give you guys a hand in doing what you want. Basically though, you can edit those lines to bind any control you want to the mouse wheel. If you don't know the syntax, bind something else to that action, then lookup the syntax by looking in that file for that key, and copying it over to the mouse wheel.

Here's the quote I found in my search:

Originally Posted by http://www.tribalwar.com/forums/showthread.php?t=390019

This is probably OFN to those that have dug into the controls.con file with notepad and made adjustments to their keybinds directly. The mousewheel defaults to the next-weapon, previous-weapon menu on the right which is painfully slow and if you’re using it as your main weapon select system chances are you’re getting owned. Some people use the numbers across the top which is also subject to problems of striking the wrong key at the wrong time, forward strafe and try to select you knife.

My Documents\Battlefield 2\Profiles\0001 is where your primary controls.con file is located, open it up in notepad.

Down towards the bottom you'll find the default mouse wheel assignments:

ControlMap.addAxisToTriggerMapping c_PINextItem -1 IDFMouse IDAxis_2 0
ControlMap.addAxisToTriggerMapping -1 c_PIPrevItem IDFMouse IDAxis_2 0

The first one is mousewheel up, second one mousewheel down

Cut the first line and paste under the heading at the top of the file labeled:

ControlMap.create InfantryPlayerInputControlMap

paste it under the line:
ControlMap.addKeyToTriggerMapping c_PIWeaponSelect1 IDFKeyboard IDKey_1 10000 0
ControlMap.addAxisToTriggerMapping c_PINextItem -1 IDFMouse IDAxis_2 0

now change it to read like so:

ControlMap.addKeyToTriggerMapping c_PIWeaponSelect1 IDFKeyboard IDKey_1 10000 0
ControlMap.addAxisToTriggerMapping c_PIWeaponSelect1 -1 IDFMouse IDAxis_2 1

We've now assigned the mousewheel up action to select the knife directly, changing the 0 to a 1 at the end tells the game that this is your secondary key/button assignment. You can do the same thing for the mousewheel down action only assign it to the pistol.

ControlMap.addKeyToTriggerMapping c_PIWeaponSelect2 IDFKeyboard IDKey_2 10000 0
ControlMap.addAxisToTriggerMapping -1 c_PIWeaponSelect2 IDFMouse IDAxis_2 1

Happy knifing and pistol sniping, or assign whatever weapon you want to this fast switch method.