Customers:
SD_CUSTOMER_MAINTAIN_ALL – quite powerful, but has no RFC (should be wrapped for it)
BAPI_CUSTOMER_CREATEFROMDATA1 – much simple, but has RFC (a BAPI).
As an additional way – using IDocs (DEBMAS).
Vendors:
Class VMD_EI_API – methods Initialize and Maintenance
Functional group LC01.
As an additional way – IDocs (CREMAS).