adding secondary sub-board to pin-out header
The simpliest way would be soldering wires on each pin. Use only single strand (monolithic) wires.
You can make use of prototype board, cut to desired size, to solder a female connector and a male connector in the appropriate direction to connect the original boards and then add the wires to the secondary sub-board. It will look more solid. But maybe not necessary.

Note that there is no garantee that the secondary sub-board will be recognized by the system or will work properly. But that already another topic.