read the quote below.. you can add this on NPCs and item scripts..
in your case for example, you can add the setlook/changelook function in the OnEquip area of the Valkyrie Helm entry in item_db into Apple o' Archer
5171,Valkyrie_Helm,Valkyrie Helm,5,100000,,1000,,5,,1,0xFFFFFFFE,2,2,256,,0,1,225,{ setlook 4,72; bonus bMdef,5; if(isequipped(2357,2421,2524)) bonus bAllStats,1; },{},{}
You can also work on it via client side, if you're going to add more items, just copy the resource data(idnum2itemdesctable, itemresnametable, etc.) then change the sprite ID in the item db, that way it would look like a Valkyrie Helm in your inventory but it would display AoA when equipped.