How to add an item to a checkboxlist in Javascript?

Hi all,
Scenario: If there is a dropdownbox having the three values: "Add, Subtract, Multiply" and there is a Checkboxlist with the same 3 values. If the user selects "Add" option in the dropdownlist, then the item "Add" in the Checkboxlist will be removed by the below code. If the user selects "Subtract" option in the dropdownlist, then "Add" item should be added to the checkboxlist and "Subtract" item needs to be removed.

I am not able to add the item back to the checkboxlist.

If I use the following code, I would be able to remove Checkboxlist item whose value is equal to the value selected in the DropDownList:

var chkObj = document.getElementById('CheckBoxList1').childNodes[0];
var dropDownListObject = document.getElementById('HTMLDropDownList1');
var dropDownListSelectedValue =  dropDownListObject.options[dropDownListObject.selectedIndex].value ;
for (i=0;i<chkObj.childNodes.length;i++)
                                   {
                                      if(chkObj.childNodes[i].innerText==dropDownListSelectedValue)
                                       {
                                          chkObj.removeChild(chkObj.childNodes[i]);
                                   //    chkObj.childNodes[i].appendChild();
                                  //     I tried appendChild, however it is not working.
 
                                       }

I would like to know how to add the item back to the Checkboxlist?

Any help would be greatly appreciated.
Many thanks,
DreamChaser