You can add wargoals through the diplomacy screen, assuming that you have a positive war score. Switch to view the ongoing wars, click on the country you wish to place an additional wargoal on, and you should see a small tab "Add Wargoal" above and to the right of their current infamy, war exhaustion, and land and naval unit counts. Be aware that each wargoal may create additional Infamy, which you need to keep in check in order to maintain healthy relations with other countries.