There is no “official” recommended way of using Bootstrap + SmartMenus in WordPress at the moment so I have taken a note to think about this and maybe add a tutorial in the docs.
But to try to help you out right now, to be honest, I am not familiar with this class (if this is it – https://github.com/twittem/wp-bootstrap-navwalker) but at first glance it does look good and simple to use. So I will spend some time testing this tomorrow morning and will let you know if it’s fine to use it. The only issue might be that the SmartMenus Bootstrap addon requires just slightly different HTML code from the default Bootstrap navbar code but, hopefully, it will be possible to modify the code the navwalker class outputs in the proper way.