If they are text files (or can be converted to text files), then you can use Endmember Collection->Import from Ascii File. Then you can save the imported spectra as an ENVI Spectral library file. ENVI does not save files to the ASD format but it can read them. When you import the spectra, you have the option to include the units (nanometers, micrometers, etc) and scale factors if necessary.

I tried using end member collection for text files but its not working. its giving error that "file is not end member collection file". broadly my problem is that I am having a hyprspectral image and trying to perform target detection in it. I have data which contains spectral signatures of targets, but this data have three type of files '.asd' '.txt' and '.000' . I am able to run detection in envi with '.asd' files but not able to use '.txt' and '.000' files. and hence unable to process/detect some targets .please suggest how to process '.000' files or use txt files for detection. thanks.

Could you post the error message you receive? The Endmember collection does not have a 'file type'. You use it to import from various sources including ascii. Did it give an error saying it could not read your ascii file?