With this script you can use a different items.xml for the Singleplayer than for the Multiplayer.
To be able to use the Script you have to adapt the xmlTag "map" in your modDesc.xml:
<map id="SampleModMap" className="ModMap" filename="yourPath/ModMap.lua" configFilename="maps/mapDE.xml" defaultVehiclesXMLFilename="defaultVehicles.xml" defaultItemsXMLFilename="defaultItemsMULTIPLAYER.xml" defaultSingleplayerItemsXMLFilename="defaultItemsSINGLEPLAYER.xml">