Thank you for your post. Our tests show that accessing a message via the CommonmessageID property results in a read-only instance of the message. This appears to be consistent whether you access the message via Multi-login, proxy or directly (i.e. by logging into the account directly). This is very likely the reason why you cannot add a custom defined field to the message.
We appreciate that it is frustrating to have project development stall due to an apparent technical roadblock beyond your control. With this in mind we always do our best to be responsive to Formativ specific questions. However, as you may be aware, your last question relates directly to the Novell Object API. While you can access the Object API using Formativ, as you can from any other compatible development environment, it is not part of Formativ. If you require time critical answers to Novell GroupWise Object API programming questions, you may wish to investigate the support options offered by Novell. See http://developer.novell.com/wiki/index.php/Developer_Support for details.