Feeds Tamper plugin to import taxonomy terms with hierarchy support. It creates the taxonomy terms if they don't already exist, respecting the term hierarchy.

In your CSV column, the taxonomy term data can be in this format: "Term1 > Term 2 > Term 3". The "Term 3" would be set to the corresponding taxonomy term reference. "Term 1", "Term 2" and "Term 3" would be created if they didn't already exist.