Greetings, Jürgen:
I wanted to let you know I retook this LSMW object and following you advice, I simplified my legacy files and I was very pleased to see SAP does all the table joins for my multiple vendor master files.
Since my legacy system is another SAP system, it was very convenient using the field auto-mapping feature and letting the system pick the right field based on the technical name. My legacy data extraction was as simple as just using t-code SE16N to extract data and LSMW took care of the rest.
I do appreciate your elaboration on this case, which actually helped to have all this nailed.
Thank you.