You will find that it is perfectly acceptable to use pecans in banana nut bread even though walnuts are typically used. If you prefer the taste of pecans to whichever type of nuts are called for in the recipe, there is no reason whatsoever why you cannot substitute one for the other.
Some people follow their recipes to the letter while others are comfortable with making some of their own changes. Although this does not always work for every recipe, pecans are a type of nut like walnuts or anything else, so you should be able to get away with using them in a banana nut bread recipe with no problems whatsoever.