The requested customization is not supported by Telerik, it is not easy to be implemented and we do not have a working example for it.
If you would like you can try to implement it yourself by following as a base the instructions provided in the following help article
as well as the code of the MOSSLinkManager command located in the wpresources\RadEditorSharePoint\5.x.x.x__1f131a624888eeed\Resources\
Telerik.Web.UI.Editor.CommandList["MOSSLinkManager"] = function(commandName, editor, args)
{
var params = editor._dialogParameters;
var docEditor = editor.get_document();
var configObj = new AssetPickerConfig("");
configObj.ClientID = editor.get_id();
configObj.DefaultAssetLocation = "";
configObj.DefaultAssetImageLocation = "";
configObj.CurrentWebBaseUrl = params["CurrentWebBaseUrl"];
configObj.AllowExternalUrls = params["AllowExternalUrls"];
var bInserting = false;
var elemToReplace = null;
var linkAsset = new LinkAsset("");
var innerLinkHTML = null;
linkAsset.ManageLinkDisplayText = false;
var frameElementId = (editor.get_contentAreaMode() == Telerik.Web.UI.EditorContentAreaMode.Div) ? editor.get_contentArea().id : editor.get_contentWindow().frameElement.id;
var elemLink = FormJSRTE_GetNearestContainingParentElementOfTypes(frameElementId, editor.getSelectedElement(), new Array("A"));
if (!elemLink)
{
bInserting = true;
elemLink = docEditor.createElement("A");
if (elemLink == null) return;
var editorSelection = editor.getSelection();
if (editorSelection.isControl())
{
elemToReplace = editor.getSelectedElement();
}
else
{
innerLinkHTML = editorSelection.getHtml();
}
}
else
{
innerLinkHTML = elemLink.innerHTML;
}
configObj.ReturnCallback = function(newAssetUrl, newAssetText, currentConfig, dialogReturnedData)
{
if (dialogReturnedData)
{
var linkText = "";
if ($telerik.isIE)
linkText = elemLink.innerText;
else
linkText = elemLink.textContent;
if (innerLinkHTML && innerLinkHTML != "")
{
elemLink.innerHTML = innerLinkHTML;
}
else if (linkText == "")
{
if (linkAsset.AssetText && linkAsset.AssetText != "")
{
linkText = linkAsset.AssetText;
}
else
{
linkText = linkAsset.AssetUrl;
}
elemLink.innerHTML += linkText;
}
if (elemToReplace != null)
{
var parentEl = elemToReplace.parentNode;
elemLink.innerHTML = "";
parentEl.insertBefore(elemLink, elemToReplace);
elemLink.appendChild(elemToReplace);
linkAsset.FixupIconsAndEmptyAnchors(elemLink);
return;
}
else if (bInserting)
{
var selection = docEditor.selection;
if (selection != null)
{
var textRange = selection.createRange();
if (textRange != null)
{
var text = textRange.text;
if (text && text != "" && !text.match(/^\s+$/))
{
textRange.execCommand("Unlink");
elemLink.innerHTML = textRange.htmlText;
linkAsset.FixupIconsAndEmptyAnchors(elemLink);
}
var tempDiv = docEditor.createElement("DIV");
tempDiv.appendChild(elemLink);
textRange.pasteHTML(tempDiv.innerHTML);
}
}
else
{
//if we cannot get the selection, use the editor paste function instead
var tempDiv = docEditor.createElement("DIV");
var pNodes = elemLink.getElementsByTagName("P");
for (var i = 0, l = pNodes.length; i < l; i++)
{
Telerik.Web.UI.Editor.Utils.removeNode(pNodes[i]);
}
tempDiv.appendChild(elemLink);
editor.pasteHtml(tempDiv.innerHTML, commandName);
tempDiv = null;
}
}
else
{
linkAsset.FixupIconsAndEmptyAnchors(elemLink);
}
}
}
linkAsset.LaunchModalDialogOnElement(configObj, elemLink, true, !bInserting);
};