Can I add rows to Output Buffer in SSIS Script Component in PostExecute?

I have a Script Component where I buffer all the rows, then do some processing, and then I want to create the output rows. I tried accessing the Output Buffer object in PostExecute but apparently that's not possible? Gives "Object Reference not set to an instance of an object" error when it hits AddRow(). Is there a way to do this?

 public override void PostExecute()
{
    base.PostExecute();

    //processing

    foreach(ChartValue cv in chartValues)
    {
        Output0Buffer.AddRow();
        Output0Buffer.usedcl = cv.Centerline;
        //etc
    }           
}

Thanks Kelly for your example. But like H B said the base.Input0_ProcessInput() calls Input0_ProcessInputRow for every row. And in my case Buffer.EndOfRowset() is false always. So I make the shorter code:

public override void Input0_ProcessInput(Input0Buffer Buffer)
{
    base.Input0_ProcessInput(Buffer); // operate rows in while loop

     //when done collecting all rows, do calculations
    CalculateResults();
}