I think what you will want to do is to go to Spectral -> Spectral Libraries -> Spectral Library Builder and use that to build a spectral library using your ASCII files as inputs. After you do that, go to Spectral -> Spectral Libraries -> Spectral Library Resampling. In the parameters dialog you see next, choose Pre Defined Filter Function and then select the ASTER sensor. Set bad values to whatever you want, give it an output file, and you should be done! Hope that helps, Jeff