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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.
 
This is why I like to stake a claim to a state prior to declaring war - the infamy hits are spread out more between when my forged cause is discovered and when I demand the second state in war. Sometimes, I can squeeze in a third state if the timing is right, but that's really pressing my luck and leaves me where I have to cool down my expansion for a significant time. Therefore, I don't press my luck unless it's for a great opportunity or to fix a particularly ugly border.

The "ugly border" thing is how I role-play most of the time. Is a war with my neighbor necessary to make for a more aesthetically pleasing map? ABSOLUTELY! FORWARD, FOR GOD, KING, AND PRETTY BORDERS!!!
 
ABSOLUTELY! FORWARD, FOR GOD, KING, AND PRETTY BORDERS!!!

I feel your pain so much. Its even worse for me in CK2 where I have to fix vassals borders as well. But at least there I don’t have to care about Infamy. In Vic2 I often have to live with disgusting borders because Infamy is to high.