Overriding type of existing property with Enumeration

I've read these resources about external enumerations:

http://blog.schema.org/2012/05/schemaorg-markup-for-external-lists.html
https://www.w3.org/wiki/WebSchemas/ExternalEnumerations

The examples in those resources show how to use external lists with an
example of the "Country" field (which is an object type) for "Movie" types.
I want to do something similar for the "employmentType" property of
"jobPosting"; however, "employmentType" is a primitive rather than an
object ("Values expected to be one of these types: Text").

What I'd like to do I think is define an externally-hosted schema for
"employmentType" and override that existing property on "jobPosting". I
could create a new property like "employmentTypeObject" or something that
uses "rdfs:subClassOf" the original "employmentType", but that seems really
convoluted. My reason is that I want to use an "Enumeration" as the
expected value for "employmentType", rather than a primitive (text string).

What's the best way to handle this? Is there a clean way to override an
existing property in an existing class?