ChrisPE already uses winpeshl.ini to start the selected shell - and may also contain other entries depending on which scripts/options have been selected in the build. For now you can manually edit this file after successfully completing a build. Edit the file (\ChrisPE.Files\ROOT\Windows\System32\winpeshl.ini) and then use the Post Config > Advanced Options > Inject ALL Files option to add the modified file and any other files manually added to \ChrisPE.Files\ROOT\ to boot.wim. You may want to (re)build an .iso afterwards? Use the Media Creation > Create ISO script and press the small Run Script button to the right of the title bar.