How do I make the import use the "key" feature to update existing test-cases?

In a way, yes, but it's also a positive.  If it were possible to update test cases, we could alter the scripts of tests that had been executed, and potentially misrepresent test results.

What I would like to see is a way for us to update test cases using the import function up until the point we flag that test cases are in a ready state. The Test Case Status flag seems designed for this purpose, and I guess it was originally in the design but hasn't been completed for whatever reasons.