Its just like how you add any custom items. but only drop sprite is required. Also you need to change the id of the zeny/cash because I think the id you used is already in the item_db.txt. https://github.com/rathena/rathena/wiki/Custom_Items#defining-items-clientside-renewal-clients-
Check this for the clientside item addition. (You don't need the male/female, only the drop and texture(item/collection) ).
Or you can just use the coins sprite in itemInfo like this :
//This is same sprite like Bronze Coin
[ZenyItemID] = {
unidentifiedDisplayName = "Zeny",
unidentifiedResourceName = "µ¿È",
unidentifiedDescriptionName = { "" },
identifiedDisplayName = "Zeny",
identifiedResourceName = "µ¿È",
identifiedDescriptionName = {
"Zeny Item Description.",
"^FFFFFF_^000000",
"Weight:^006600 0^000000"
},
slotCount = 0,
ClassNum = 0
},
//This is same sprite like Mithril Coin
[CashItemID] = {
unidentifiedDisplayName = "Cash",
unidentifiedResourceName = "¹Ì½º¸±È",
unidentifiedDescriptionName = { "" },
identifiedDisplayName = "Cash",
identifiedResourceName = "¹Ì½º¸±È",
identifiedDescriptionName = {
"Cash Item Description.",
"^FFFFFF_^000000",
"Weight:^006600 0^000000"
},
slotCount = 0,
ClassNum = 0
},
Change the "ZenyItemID" and "CashItemID" to the one you use in extended vending system.