How to get the flavors to meld when I add fruit and sugar to plain yogurt?
To sweeten it, add your favorite sweetener: sugar, honey, agave... whatever you prefer. This is important, because sweetness will help bring out the flavor of the fruit.

General advice:

If you have a fruit you want in your yogurt, pick the form (chunks, pureed, mashed, juice) that you want and mix it in. The Chobani yogurts appear have a variety of these forms, always along with some sugar. If you want banana flavor, sure, mash up a banana and mix it in. Or if you want chunks of fruit, chop it up. Assuming the fruit is soft enough, for this purpose, mashing with a fork  is probably a good substitute for pureeing; you'll get some small chunks and some juice.

In response to the updates:

if you want actually fruit-flavored yogurt, then you need juice and/or puree. If you don't have any other tools, mash it as best as you can. If you have a small food processor or blender that will work with small enough quantities (or are making big batches), blend things up and stir them in. Juice and puree will disperse quite well through the yogurt.

Bananas mash easily, especially ripe ones, so just do that with a fork. Strawberries aren't as soft, but you should get enough juice to flavor with if you mash them. Pineapple is tougher. If it's juicy enough, you can probably get some juice out by mashing it; otherwise you might be better off with a food processor or blender. (If it's from a can, you can use the liquid from the can, too - that might also have sugar in it.) Depending on how thorough you are, you may want to use the juice and discard the fibrous parts that are left over.