Per MSDN, The BizTalk Adapter does not support has several limitations, including a lack of support for ITAB II table types (hierarchical tables):
The SAP adapter does not support RFCs with complex parameter types, including ITAB II (hierarchical) table types. The SAP adapter [also] does not support RFCs having custom ABAP types.
I'm curious if anyone has come up with a workaround for this issue, or if it would require writing wrapper RFCs.