Thanks for your answer. I actually wanted our customer to be abble to create sheets on a non-published application (after dupplicating), republish over our application and keep their custom sheets when we deliver a new one.
Doing it this way, they have acces to the fields in the left panel, which is not possible when editing a sheet in a published app (only the MasterItems are visible). However, I found out that you can access all the fields when creating a graph or a formula, so this is not really an issue anymore.
Though it would be nice to be abble to acces the whole field list in the left panel on a published app, we can live with it for now.