You “could” use the Master Item List for your engrams - if you do the item will work only until the parent company updates next. Then that update will overwrite your information - or worse, your add-on item will be in the place where something they put is in place, causing one of the two to error out. It’s just not a good idea to use that list, it’s not meant for us.
Be safe - Be clean - Use the Additional Item Lists (Engrams, Structures and Classes). You’ll be happier for it.
Now to answer the question: Can you hide an Engram?
Sure - Don’t create it in any list. Have the admins use the spawn item as mentioned above.
I have many consumables that are not engrams - and are crafted in special crafting ways. Like for example the Dry Kibble - Universal Dino Food, great for feeding (All dinos prefer it over normal and it has a long shelf life (Even longer in a feeding trough) - terrible for taming. It’s made with no eggs so you can save them for baby dinos. Shameful Plug You can’t learn it - it’s an auto craft in the Kibble Maker, BUT an admin could spawn the item still using the full cheat command.
Hope those ideas help.