If you loaded an ISO file directly into NTLite, there isn't a way to import your own answer file.
The older method is to extract the ISO's contents to a local folder, and have NTLite load the folder instead. Using this method you can copy your own autounattend.xml to the extracted folder's root level. When NTLite creates an ISO, it will include everything (including your answer file).
To copy your own answer file to \Windows\Panther, load the image first and copy the file into the mounted directory.