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).