Create a WorldItem on server start

Hi guys,
this week i've spent all my time on making rust plugins
And today i want to make a plugin that will consist to change an item on the server by another.
I was testing some code then i found that when you can change a WorldItem ItemId , if for exemple you change the ItemId of the WorldItem RifleBody to the ItemId of SmgBody then you ServerUpdate it, the RifleBody will change to SmgBody.
So the problem here , the only way i can get the WorldItem Entity is from Hooks like OnEntitySpawned().
Wath i want to do is to initialise for exemple a WorldItem RifleBody , then change it & update it without needing any external hooks that will return a BaseEntity or BaseNetworkable
Thanks you