Not sure if you can...think the option is siren only, siren and horn
Though if you unplug the power sounder / siren...it should just use the horn though probably will kick up an error on the SID....
hmmm reminds me...need to sort an air horn for the saab...as forgot to take my vintage air horn off the vectra...now that was loud...any animals stepping out onto the road used to flee from it (Cats/pheasants mainly) standard horn...not so much...then again it was a 1980s Stebel....and kicked out ~139Db at something like 3 metres....