Resolution:
TIBCO Adapter for Siebel  Publication Service can be configured to publish some extra fields not already present in the imported schema using the following workaround
1) You could have the extra fields added to BC itself and this will be included in the published message, if it is also selected while importing the BC schema.

2) If the Publication service is configured with SendCompleteIntegrationObject operation, the following workaround can be used:
----------
Workaround to publish an extra field in the adapter message:
-----------
--> First configure the adapter Publication Service with SendCompleteIntegrationObject operation.
--> Fetch the required the Intergration Object schema and configure the adapter schema.
--> Browse to the adapter schema under AESchemas folder.
--> Add a new scalar field like "time" of String type.
--> In the HTTP Request, add this field as well to the SiebelMessage.
--> Send the event to the adapter.

As observed that the adapter publishes this field as well.
Attached here is a sample project for reference.

How do I get extra fields to be published by Siebel Adapter Publication Service?