If you have a time series and you want each classification file as a separate band in your final image you would could use b1 +b2 + b3 +... and assign each class image to a band. This is on the assumption that each class image is the same in size and they cover the exact same areas. If they don't you could try layer stacking found under Basic Tools on your main menu (ENVI 4.x and ENVI Classic). If you are using ENVI 5.0 look in the Toolbox under Raster Management.