Hi,NLH,
Not really.
You have to bind your EC with your new custom BO.
You can't change already existing data of a standard BO with an EC.
If you want to change a standard master data, you just edit the standard record (that you want to) from Edit option on the Cloud.
If you want to add some fields to a standard form, use extension fields. You will have read and write access for those fields.
Creating an EC is used for some other purposes(for example -->when you want to add a table to a standard screen which is somehow related to the standard BO).
When the standard BO is saved, yours is getting saved either. Your EC will act as a QA of your custom BO. All the event :AfterModify, BeforeSave,OnSave validation of the custom BO will run on EC as they run on QAF or OIF.
Hope this helps.
Fred.