Unfortunately, you can not add custom fields to a composing/draft message. However, you can add when the message is saved (i.e. stored in GroupWise database). You can save draft message using ItemSaveMessageDraft() token to save a draft copy of the current message in the specified folder. If no folder is specified, the user is prompted to indicate which folder to use.
See the following thread for more information: http://www.advansyscorp.com/forums/topic/2131098961/
We haven’t investigate X-Field on composing message, please let us know if you have any success.