If the bands are already co-registered but simply reside in separate files then you can use File ->Save File As ->ENVI Standard and import all the of of the files into the dialog and save to a new file. Alternately, you can save it to an ENVI meta file, which creates a text file in the directory of the image files that points to each dataset and opens them as one 'mata' file. The meta file can be processed in ENVI like any other file. If the data are not co-registered and of different dimensions and/or pixel sizes, then you would need to use Layer Stacking to stack the images together. This option will use the first image as the 'base' and all subsequent images will be resampled to the base. This option will only work for bands that have map projection information. I hope this helps. - Mari