User can add field value
I will try and provide you a general direction which I think might serve you well:


Create a text field, with label something like this "Please add tags of your choice if you dont find adequate options".(purpose is explanatory from the label)
Create a content type, "Proposed Taxonomy Terms". Let it have only title and body field.
Now on saving a node of the content type in which we had the suggestion field, grab the value of the proposed taxonomy terms from it, create a node of type "Proposed taxonomy term" and clear the value of the field in the first content type.
Now the nodes in the content type "Proposed taxonomy Term" will be your basis which you need to set up such moderation. it will initially remain unpublished. Upon validation, write custom code, to grab the title on its save and create a taxonomy term in the vocabulary(which was used as term referance field in the first content type), and delete the node (or you may choose to keep it). 


So next time when user tries to create the node of the first content type, they will have it as an option. this is a complex workflow. The logic parts you can do custom code better, employing drupal hooks or you may choose to use rules module (not sure it will serve the purpose exactly though).

Hope this long answer did not confuse you.