How to add a typed value to an existing value in a cell in Excel?

Question

When typing a numeric value in a cell that already has a non-zero numeric value, is there a way to add the value to the existing value instead of overwriting it? For example, if a cell has a value of 4 and I want to add the value of 2 to the existing value and replace the cell's value with the sum of 6, is there a way to do this directly in Excel? It’s trivial in the example, but the larger the value, the more error-prone the in-my-head procedure becomes. I'm looking for a simple keystroke solution, not a formula that references another cell. Nor do I want to reference a VBA macro as that just adds more keystrokes.