Banana wine is really pretty good. After fermentation, the original sweetness will be gone. Depending on the yeast you used, it may finish completely dry. After it's done, you could backsweeten to taste. I'd stabilize with campden tablets (sulfite) and sorbate before adding any sweetening.
How you do it:
After fermentation is complete, you'd add 1 campden tablet per gallon, crushed in a little boiling water. Mix that with 1 tsp sorbate per gallon. Add that to your wine. Wait a few days and then make a sugar syrup by boiling 1/2 cup water with 1 cup sugar in it. When it's clear, let it cool. Put some wine in a glass and gradually add some of that syrup to taste. When it's right, take a s.g. of the sample. Then just sweeten your whole batch to that s.g. That's the easiest way because then you don't oversweeten your whole batch.
I'm going to be doing banana wine this fall when I can't get other fruit. I didn't think about putting orange juice in it- that's a great idea!
Lorena