Hi Eric, do you mean that values from other sources that have to be requested directly (that is, not through Lightstreamer) should mix seamlessly with data coming from Lightstreamer?
I'm afraid there is no facility for this task, as Lightstreamer library for Silverlight only provides update notifications and does not cover the visual level;
moreover, its internal "model" level only depends on the subscriptions and cannot be altered in a different way.
Hence, you should implement your own data model, which gets contributions from both sources.

On the other hand, does your external data source play at the "control" level, by affecting the current subscriptions?
Subscriptions for single items can be added and removed at any time, provided that the SubscribeItems method rather than SubscribeTable is used for subscriptions.
If only columns have to be added to a current subscription, then doing an unsubscription and a new resubscription cannot be avoided.

May you please clarify about the connection exception question?
Do you mean a PushConnException upon a call of OpenConnection
or an OnFailure notification to the listener on an established connection?