I don't understand what you are asking. Do you want to enable the "Custom Fields" on Catalog mode?
But this doesn't make any sense, the custom fields will be useless because product can't be added to cart, because "add-to-cart" button won't work.
You can do this:
Don't enable the "Catalog Mode", but hide the "Add to cart" button by this method:
- open this CSS file:
and include this CSS rule:
so, even though you are not using the "Catalog Mode" the "add to cart" button will be invisible, but the "Custom Fields" will be visible.