What could melt the slick, seemingly frosty demeanor of a stoic storm trooper in ‘Star Wars’? Well, for starters, Ryan Morris, the general manager of Venkman’s in Atlanta, thinks one must start with rum. Of course! Sometimes, when the weather calls in the balmy South, he likes to run this tropically tweaked tipple through his on-premises slushy machine. But you can shake it up, too.