All your data is in column A (beginning at row 1).
In column B, row 1, enter =A1&","
This will make cell B1 equal A1 with a comma appended.
Now select cell B1 and drag from the bottom right of cell down through all your rows (this copies the formula and uses the corresponding column A value.)
Select the newly appended data, copy it and paste it where you need using Paste -> By Value