Write a default value inside a given cell in C #

I'm doing a console application that reads data from a worksheet and compares it with values entered in the database. There is a default value, if a field in this worksheet is not filled, I need to get the value of the bank and set it in the worksheet in the cell that was not filled. My problem is how to set this value in this cell. I can do in a new cell.

public class EscreverPlanilha {
    public void EscreveValorDefault(Campo campo, string column) {
        var str = new StringBuilder();
        str.Append(campo.ValorDefault);
    }
}