You cannot add an external force to a wedge in Unwedge.
There are ways, however, to approximate an external force. If the load is vertical, you can increase the unit weight of the material. If the load is normal to the excavation face, you can add a negative support pressure load. If the load is in some arbitrary direction, you can use a seismic load to mimic this. The seismic load is directional. Given the force magnitude, divide it by the weight of the wedge to get the seismic coefficient.