It is not that simple.
First, implement owner draw mode of the string grid. Do all of the drawing of the cells in your code and when you have that perfected then you use the same code in the draw method to repeat the process except you draw to the surface of a bitmap instead.
Create a double for loop X,Y so you can scan the complete cell count and for each cell you execute the same code you did for the owner draw except draw on an image instead.
At least this way you can get all of the cells , not just those that are visible.