Just bouncing around some ideas here...
You could get a kit for a regular cream ale, and sub in a hefeweizen yeast strain, the yeast would likely impart some of the fruity banana esters that are typical in the style.
You could also look around and see if there is a banana extract or flavoring syrup that you could add near the end of primary fermentation.
I've never used whole banana before, I've used other fresh fruit. It can be pureed in a blender or food processor, heated on the stovetop to pasteurize (stir constantly or you can scorch/carmelize the sugars in it) cooled, and added near the end of primary or into a secondary.
Just curious, what's your base recipe? I've got an all grain cream ale that gets rave reviews from everybody who tries it.