Hi,
an interesting approach. Usually, an ECC system does not need to have all users. Anyway, LDAP connector is what you are looking for. SAP uses it to export personal records into LDAP server for one of the IdM scenarios. You can have a look there for an inspiration. The LDAP server in that case is not AD so you might need to do some additional mapping. The program used by IdM scenario should be RPLDAP_EXTRACT_IDM.
I would also suggest to think if having a dedicated system for user provisioning (e.g. SAP IdM) isn't a better idea for future than implementing a custom solution in ECC.
Cheers