This is an answer:
function addColumns(){
var ss = SpreadsheetApp.getActiveSheet();
var len = ss.getLastRow();
for(var i = 1 ; i < len +1 ; i++){
var num1 = ss.getRange("D"+i).getValue();
var num2 = ss.getRange("E"+i).getValue();
ss.getRange("E"+i).setValue(num1+num2);
ss.getRange("D"+i).clear();
}
}
It is not at all efficient ... so you will find it strangely slow. But it works.
I agree with @eddyparkinson that it is good to learn google apps script (which is pretty much the same as javascript) because you are likely to get a lot out of it. I had to start learning it from scratch about 1 year ago, mainly by looking at this particular forum for google apps script and then, when I got stuck, referencing the huge quantity of javascript general help you can find on the internet just by doing a search for the term in hand.
EDIT
To make it apparent, I have shared this script within a spreadsheet (click here).
In that spreadsheet, there is a custom menu called "Script Center Menu>Add column D to E
This will then add all the values in column D Sheet1 to those in column E Sheet 1, displaying them in column E and deleting the original value in column D. (If you wish to experiment, you can prepare the columns' values manually, simply by typing your required values into columns D and E - it may be easier to first clear current values in columns D and E. An alternative way to set yourself up the values for a demonstration is to use the second menu item Script Center Menu>Copy Columns which will simply copy the existing values in Sheet2 over to Sheet1).