I guess I am not sure what you want to do.
If you want the display to change but have the cell not change you could do
something like in column b
=if(C1<>"",C1, Calculation)
This would let someone effectively write over the display in Column B by
entering something in column C
If I am reading what you are saying correctly, you want to be able to Enter
something directly in a cell but have the cell still have the original
equation. I really do not think this is possible. But hopefully there is a
wizard out there would can prove me wrong.