How to modify element's data during import?
The EVENT_STEP_BEFORE_ELEMENT_SAVE is actually fired after the content has been applied to element attributes and field content.

Instead, you'll want to modify the element's content directly. You can do this via:

$event-&gt;element-&gt;productDescription = " - blah - ";  


As the plugin author, I may change this so it acts as you've suggested. Currently, modifying $contentData essentially does nothing. However, this will require a change to the event where it passes in attributeData and fieldData separately, as they need to be applied separately back on the element. I'll look at adding this in the next version.

Let me know if that doesn't work!