Hi Howard,
is the "Nielsen ID" a customer field in the ECC business partner or a standard field of an industry solution?
In case it is a customer field, you could check the enhancement guides concerning CIF if they help you to transfer that field:
Interfaces - Core Interface (CIF) and BAPIs - Supply Chain Management (SCM) - SCN Wiki
Enhancement in /SAPAPO/MAT1(it's for material master, but there is a similar concept for BPs)
Best regards
Patrick