Where you would add a shot of vodka or something, you would add about 4oz of prepared kava at your desired strength. Then you mess with the ratios of the extras to get the taste close. Creamy and tiki style drinks work very well with kava. You can also make your own non alcoholic spiced rum and other stuff like that with herbs/spices steeped for awhile. you can do a google search for whatever flavor add on that usually has alcohol and there will likely be something.
Also, if you're looking into kava cocktails. Some of the BEST kava cocktail stuff i've had has been using lacto clerification. The theory behind it is that you add a lot of a base like milk or coconut mlk, add a acid like lime/lemon. Let the whey separate out then strain using a coffee filter. You end up with a super clear and very clean tasting kava cocktail. It has 0 earthyness and a tiny bit of mouth numb with just about the full effects of whatever u put in there. They're a pleasure to drink.