BP custom fields replication CRM to ECC .

Maybe notes 831536 and   736595 will help. (The X on the right-hand side of note 736595 indicates it's valid for subsequnet releases.) See also note 833585.

While creating custom field through EEWB ; did you enhance the R3 side too? If not, re-do you EEW and make it by enhancing the R3 side too (otherwise it will be quite difficult for you - since you dont know how to debug from CRM to R3).

Once its done, you will find the custom field in the ECC master data table. So, along with your BP creation, the custom field also get filled (SAP will take care of it if you do the EEWB properly).

Now, if you want to just use that field in ECC, you can access it through any program or FM. But, if you want to show that in ECC side BP screens, then you need to do screen enhancements (or similar) and get it done.