Is there any way to take a preexisting field with content, and generate taxonomy from it? I have a content type called 'classes' in the content type is a text field called 'instructor'. I also have a separate content type called 'instructor profile'. The two content types 'classes' and 'instructor profile' are not tied to together in anyway unfortunately (not setup by me). 

I would like to tag the existing 'instructor profile' content type with their 'classes'. So for example, if you are looking at the instructor profile, you can see what classes he/she teaches. The easiest way to do this would be take the existing 'instructor' text field under 'classes' and generate the vocabulary so I can match those up with "instructor profile' under a view later on by tagging the Instructor's profile with the right vocabulary. 

However, I can't seem to find a module that will take an preexisting field and generate terms from it (or convert that field into a free-tagging taxonomy term). Any idea?
It sounds like a job for an entity reference field

For example, add a "field_classes" entityreference field to "Instructor profile" referencing the class nodes that instructor teaches.

Alternatively, add a "field_instructor" entityreference field to the "Classes" content type, referencing the instructor profile(s) for that class.

Using Entity Reference, each content type can easily list the related instructors and classes.