Fill Mail Merge document up with subform fields of an Inventory module record being in the Related List

Fill Mail Merge document up with subform fields of an Inventory module record being in the Related List

Hi,
I try to insert subform fields from an inventory module record being on the Related List of another inventory module record into a Mail Merge template without success. For example: we use ratecards in licensing and this ratecard items are available in a subform of a Purchase Order record. How can I insert these subform fields into a Mail Merge template in the Quote module ? I can insert the fields from the main form but not from the subform of the related module record.

Is there any way to do so?