Hi Ivonne,
Did you get a chance to view whether the data is coming for the Z fields into the Queues from ECC to CRM? Put a break point in the Middleware mapping Function Module(R/3 to CRM) in The Replication Adapter object that you are using. You can find this in your adapter object based on your replication adapter object. When there is an inbound queue in CRM in the transaction SMQ1 start debugging LUW of that particular queue. It will break at the Middleware mapping function module. check whether the Z field's data of the ECC system is coming till here. If the Z fields data exists then now again go back to adapter object. Copy the Middleware mapping function module (R3 to CRM) into a Z function module.
Now write your logic for updating the data to BUT000.
Hope this helps for your problem.
BR,
Bhargav