We don't need sales data maintained on the customer in the ECC to replicate the contact person. As soon as user maintains the contact person on customer in ECC, it should get replicated to CRM, provided adapter CUSTOMER_REL is run through Tx. R3AS in CRM.
hope this helps... http://wiki.scn.sap.com/wiki/display/CRM/Contact+person+replication+from+SAP+CRM+to+SAP+ECC