You have to create a custom RssChannelProvider that inherits from Telerik.Cms.Engine.RssChannelProvider. Then override SetGeneralProps
methods from where you have access to the RssItem item object and you can set the item.Link.
RssChanelCustom : Telerik.Cms.Engine.RssChannelProvider
SetGeneralProps(Telerik.Rss.RssItem item, IContent content)
//(System.Uri)"format the link here";
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