If I'm not mistaken about what you are looking for, KNA1 and ADRC joined on ADRNR will get you business partners and their addresses. Is that what you're looking for? To extract them, look into either a custom program driving (possibly) BAPI_CUSTOMER_CREATEFROMDATA1. Look into LSMW for loading data from a flat file. Look at building a custom program to extract the data from the old system into a flat file either for your BAPI_CUSTOMER_CREATEFROMDATA1 driver program, LSMW, or a customer BDC program that loads the data and creates the business partners that way.
I hope that helped.
Crew