"Let's model me a mine"
M.E.G.M.S.
Mining & Exploration Geological Modelling Services
Merging a unit is equivalent to deleting an overlying unit and having the unit immediately underlying expand up to occupy the area of the deleted unit.
Simply delete the overlying unit while the Insert Mode is Overwrite - there is no special "Merge" action.
The Insert Mode does not affect Merge Mode operations.