Sorry for the late response,
First you will need to add your own item into the game, i posted a tutorial about this for Hand weapons. First do that.
Then if you want to add it to the shop just go to your > Matchserver folder > open shop.xml > and you will see this for example :
What you want to do now is this
Just add a *ENTER* between 1 of the "Sell item id's" and add a new line like :
and change the itemid (131) too the one of your item in zitem.xml.
Also make sure your iscashitem="false" in zitem.xml because otherwise they wont show in the in-game shop =)
This should do the trick.