How to manually create a menu item for a taxonomy term?
I don't know if there is a module for this, but what you can do is to create it via views. There are two ways I can see:


Install Entityqueue.



  The Entityqueue module allows users to create queues of any entity
  type. Each queue is implemented as an Entityreference field, that can
  hold a single entity type.
  
  For instance you can create a queue of:
  
  
  Nodes
  Users
  Taxonomy Terms
  



Install Flag and Views Bulk Operations, then add a bulk operation on the flag in the specific view's UI.


Make sure to cache your views for better performance.