PO creation using LSMW through batch input method

Hi, I am new to working with LSMW and i have completed a project for creating purchase orders using the direct data input method. I need to do the same using batch input method, however in the maintain structure relationship step, it is only allowing me to assign one structure to the recording that i have done, however in po creation we need separate (nested) structure for header and line item, could anybody help me past this please?