how do i add a login menu to superfish menu module?
Instead of using SuperFish, you should consider using:
TB Mega Menu

TB Mega Menu allows you to create a mega menu with an innovative
back-end user interface and synchronized with Drupal core menu.

Synchronized with Drupal core menu, auto updated if there’s any change with - -  - Drupal core menu
Bootstrap framework
Responsive ready
Multiple built-in styles designed with years of theming experience
Visual and friendly user interface - what you see is what you get
Custom style for each element of your mega menu, just input extra css class name
Multiple built-in CSS3 animated effects
Work independently with themes


Speaking from experience, I know you can add the login block to TB Mega Menu
Here is a Video Tutorial