The default implementation of MergeField supports only plain text. In order to merge with more complex data you should implement custom MergeField. More detailed explanations about how to implement such scenario you can find in our Custom Fields article
In addition, I am sending you a sample project which demonstrates how this could be achieved.
Let me know if you have any further questions.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.