This is not currently an option in the Special Importer, but would be a good thing to add.
One current work around, post import is to use an expression column.
Or use the menu option Data\Find & Replace
If you have not used the Special Import before it is accessed under Data\Import Special.
NOTE: We don’t have section on the on-line help yet on how to use the importer but there is a chapter in the DataGraph Manual (pdf under the Help menu), that has examples. The special importer can map text data to do a find and replace, but we have not added the ability to do a replacement on numeric data. We will look at adding this for the future.