Purchasing addon when data session is active

Let's say we have an user that is charged for using data because he has no data resources (realtime charging is used). Now he has an active session and there is reservation on his balance. If he now buys addon with data resources, then the whole reservation on his balance is removed and reservation on data resources for traffic in the whole session is made. When session ends, this user only has data resources (from addon) impacted, there is no charge on customers balance. Is there a way to configure BRM so, that data resources would be used only for