how to add a horn??

[icon00]

how do you go about adding a horn to the warthog in bg???

i have searched but cant find out how to do it, so could some one help, by either ppf or explanation on how to do so

thanks

edit: also how would i go about making players of different teams get in the warthog together. like it is in halo 2, how any body can ride in the hog together no matter what team they are on or if its ffa thanks