I found a solution for my case, maybe it's not elegant, but it did the trick: (PDFSharp V. 1.50.5147.0)
Here is the solution:
Code:
PdfComboBoxField cbxField;
if ((cbxField = field as PdfComboBoxField) != null)
{
cbxField.Value = new PdfString("( )");
cbxField.Value = new PdfString(MyValue); //MyValue is not required to be in the combobox choice list
}
//Notes:
// cbxField.Value = new PdfString("(" + MyValue + ")"); //MyValue must be in the combobox and it will be displayed with the parentheses in the pdf form.