If you want to overlay the multispectral data on the DOQ for visualization purposes, similar to how it can be done in a GIS program, you can use the new ENVI interface introduced in 4.3: ENVI Zoom. For this to work, your images need to be in one of a few standard formats: GeoTIFF, ENVI format, JPEG, JPEG2000, or NITF. GeoTIFF would probably be easiest. Open the DOQ image in Zoom, then open your multispectral image. The latter will be overlaid on the former in a new layer as long as there is geographic overlap between them. The "classic" ENVI interface can't do this sort of overlaying, but you can physically combine the multispectral and DOQ data using the Mosaicking tool.