Importr module allows to transform values before importing to Salesforce.

Start by selecting the Transform to update individual records or Mass Transform option to update records in bulk from the Importr dataset.

In the dialogue window, you can select the records that will be transform. Also, you can exclude or include fields that will be transformed.  

Once you made your selections, press the Transform button. 

Read more:
- Getting started with transform rulesCreating transform rules