you dont really need any converter or anything as long you have that item in your iteminfo.lua and you have a notepad ,
let's take an example , item 19156 .. as in lunar rainbow ,
its an upper headgear right ? so just duplicate one of your existing upper headgear in your item_db ,
then change the Id , AegisName , Name and ViewSprite to lunar rainbow's (make sure there's no duplicate of another lunar rainbow in your item_db)
for the item bonus script like +mdef etc , you can just copy the script from here , https://www.infinityro.it/item/view/?id=19156
or make your own item bonuses with item_bonus.md as your guide .
please take a note that hercules aint using some of "class" bonus for some reason ,
so , adding class bonus damage atk+1% as "bonus2 bAddClass, Class_All,1" wont work
if you want to add atk+1% , use "bonus2 bAddRace, RC_All,1" , for matk+1% , you can use "bonus2 bMagicAddRace, RC_All,1"