Adding content type field value to a taxonomy term? What I'm trying to say at the title is, I have this Select List field that gets its values from a taxonomy called "Thing". 

The taxonomy term are (Thing-1,Thing-2,Other).
When "Other" is selected a text field called "Specify" will appear. What I'm trying to do is when the user submits the form. Whatever he placed on the "Specify" field will be added as a "Thing" Taxonomy term. 

So the terms will be (Thing-1,Thing-2,New Thing,Other).

I might be over explaining this, but I can't find anything on google.
If you are using Select (or Other) module and it does not specifically need to be a taxonomy term, there is an option for exactly this:

On the field edit page, the &quot;Other value as default value&quot; allows you
to select how other will be handled:

Add the values to the other textfield-&gt; store the value but don't
change the list

Append the values to the current list-&gt; store the
value and add to the list attached to the node/user/entity

Append the values to the available options-&gt; store the value and add the value to
the list so that it can be selected on other nodes/users/entities

Ignore the values-&gt; do not store the value



EDIT (courtesy of DuaelFr)
You will need the 7.x-3.x-dev version to be able to use this module on a Taxonomy Reference Widget.