If you are referring to the "Map Key" option in the Annotation window, the short answer would be no. The Map Key is generally used in conjunction with classification images that have specific colors and names for each class stored in the ENVI header file. The Map Key reads these in and displays them appropriately. One potential workaround is to apply a Density Slice to a band of data, save that Density Slice to a Classification image, then use the Map Key to display the "classes". You would likely have to edit the key to reflect the value ranges used to create the density slice, though.