Enter from the left menu: Tools > Keyword Suggestions.
You'll get to the suggestions section, where you can select a URL at the top left and get keyword suggestions for each one of your keywords.
Check the keywords you want to add, than when finished click "Add" at the top/bottom.
It will take you to the Advanced Add, set the wanted tracking settings and save (Add).
Read here more about the Advanced Add.
You can get to the Keyword Suggestions directly from Manage URLs by clicking the magnifying glass icon of each URL.