GroupWise security precludes you from changing most attributes (including the body text) of pre-existing Group items. (i.e items sent to you from someone else).
You are able to change more attributes of posted (i.e. personal) items, but I suspect this is not what you want to do.
Depending on your needs, you may wish to investigate the possbility of adding custom fields to the messages instead, or storing custom data externally (a database perhaps?) and linking it to the GroupWise message via its message ID).
I hope this helps.