You can easily add a prompt window to collect the image caption text and append it to the inserted image when inserting an image through the image manager. This can be done via the OnClientPasteHtml event of RadEditor:

<script type="text/javascript">
    function OnClientPasteHtml(sender, args) {
        var commandName = args.get_commandName();
        var value = args.get_value();

        if (commandName == "ImageManager") {
            //See if an img has an alt tag set 
            var div = document.createElement("DIV");

            //Do not use div.innerHTML as in IE this would cause the image's src or the link's href to be converted to absolute path.
            //This is a severe IE quirk.
            Telerik.Web.UI.Editor.Utils.setElementInnerHtml(div, value);

            //Now check if there is alt attribute 
            var img = div.firstChild;
            if (!img.alt) {
                var caption = prompt("Enter Image Caption", "");
                        

                //Set new content to be pasted into the editor 
                args.set_value(div.innerHTML + "<br/><div style='font-style: italic;font-size: 11px;'>" + caption + "</div>");
            }
        }
    }
</script>
<telerik:RadEditor runat="server"
    OnClientPasteHtml="OnClientPasteHtml"
    ImageManager-ViewPaths="~/Images"
    ID="RadEditor1">
</telerik:RadEditor>