The AppendMode functionality for the RadAutoCompleteView is in development and will be shipped with the Service Pack (planned for 17th of October 2018).
Let me know if you have any other questions on this.
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Feedback Portal
and vote to affect the priority of the items