Suggest you move the switch while honking the horn 
Funny that DeTomaso used the "pump' icon on the horn switch (switch position #6, the second from the right). I'm sure someone on the Ghia line got a promotion for simplifying one aspect of part management in the factory...!
If you need an electric fuel pump, then Yes, my suggestion is that the power for the pump come from either position F10 or F11. But unless that switch is a kind of theft protection, you should not need a separate physical switch for a single fuel pump. The ignition switch for the Goose is simple 3 position Off/Run/Start, fuse site F12 is directly turned ON by the key when in the Run position and that connection switches the Carello relays that provide power to the F10 and F11 fuse sites. An electric fuel pump should use those fuse outputs directly or use them to switch another relay to take directly from Battery Power.
There is no 'accessory' position for the key, but almost everything else (the electric windows and even the radio) can be used with no key in the ignition (Fuses 5-9 always have power). But for example, the "Country" horn needs to have the switch ON to operate (the city horn works all the time). attached is a simplified picture of the City/country horn switching, when in City mode the Bosch horn is directly grounded thru the steering column...in Country mode, the #6 (fuel pump!) switch instead activates the FIAMM relay behind the steering wheel, which sends 12v to the compressor in the right grille area.
See pictures showing simplified diagram for the Fuse switching and Horns attached !
