Hi Tamas,
I don't think you are able to add a record to 0CUST_COMPC_ATTR while processing 0CUST_SALES_ATTR. If you want to add a record to 0CUST_COMPC_ATTR, then you have to do it here in Enhancement RSAP0001 or BAdI RSU5_SAPI_BADI.
Alternatively, you could create a Generic DataSource (e.g. Function Module) to fetch this specific group of Ship-to Parties. It will be even more transparent from a data staging perspective,
Best regards,
Sander