I think the key word here is
Originally Posted by MargotMarigolds
....with products not necessarily in the system ....
I don't think XC has been able to do this in XC4. Maybe you had some customization.
How can you add something to cart if it has no record in the store?
You can create generic product and add text attribute to it so you can fill out whatever the product description should be at time of adding to cart - that way you can treat this as product not present in the system.