Hi Eric, as said by Jeremy, users live in Netweaver UME (User management Engine) then you can actually configure UME to source its user from different sources (Java - LDAP - ABAP).
You can use an LDAP source but you can also use an Abap system like your ECC; which is what you are trying to achieve.
some links:
I invite you to read thoroughly because there are some complexities and limitations.
Regards,
Arnaud