I think that sitemap canot be created within Google Webmaster Tools, it can be
only submitted from there in case you already created/generated it from within your CMS or using some
sitemap generator.
Regarding
decreasing bounce rate, i think it is questionable. You may achieve that Google index more sub-pages if you submit sitemap, thus more visitors
may come from search engine (targetted visitors who may stay longer time on the page).