Adding new items in Essentials

Is there any legit way on adding new items of any kind into a Pokemon Essentials game?

I tried to add a new item within Pokemon Essentials, and it wouldn't function, especially evolution stones/items. Does anybody know how to fix that problem?

You'll have to add the function of the item in the Script-Editor. The easiest way would be to look how a similar item works (e.g Firestone). Just search in the Script-Editor for FIRESTONE, look in which script sections the function is defined and add your custom item to the lists.