You would first have to have the spectral response file from the data provider. This file describes the band response across the full band width. You can get an idea what these spectral responses per band look like by view one of ENVI's existing filter functions (for say Quickbird). If you have this file, then you will probably need to format it so it can be imported as a spectral library to ENVI. This format consists of the first column of wavelengths for all bands, followed by columns of the spectral response for each band (so 4 columns for 4 bands). For more information on this, it is probably best to contact Technical Support directly.