How to import Esri Raster to IDRISI with NoData value?

I made some point features in ArcDesktop to build training samples to import to IDRISI. I had 7 classes and convert the feature to raster to build the signature in IDRISI. When I import the data to IDRISI it appears with a new class of all pixels that aren't in the 7 classes. How can I import the raster to IDRISI with NoData value on the pixels outside teh 7 classes of my nomenclature?

Thanks,

Christoph