Should be pretty easy.
I'm not sure if the horn contacts in the steering wheel connects a ground or a 12v+ though, so let me see what I can find there. I would think it's 12v+, so in that case and assuming your Dixie horn also uses 12v+ as it's trigger, you'd just disconnect that wire(or cut it if you don't mind cutting stuff), run it to the middle position on a A/B toggle switch and run one of the other poles on the switch(we'll call that "A") out to the stock relay(the wire you disconnected or cut) and the other pole(we'll call that "B") would go to your Dixie horn.
When the A/B switch is in the A position, everything is normal, when it is in B position your Dixie horn will be triggered by your steering wheel.