Hi Rick,
you can customize a lot in MPP, but you will soon realize that some customization require massive changes. Does your second ItemType contain a multilevel-bom or is it a more simple ItemType? Do you want to be able something similar like building a MBOM from a EBOM with all the validation checks?
The EBOM/MBOM functionality is optimzied for Part. It analyzes the Part structure to get an EBOM. One step that happens here is that MPP breaks done multilevel BOMs to a plain singlelevel structure. The corresponding Methods is quite huge. So this use case would definitely not become easy.