The following is created user control in web application emailafriend.ascx.We can add an user control in transformation for customzing webpart.
Transformation markup:
<%@ Register TagPrefix="custom" TagName="EmailAFriend" Src="~/CMSWebParts/HomeLearningCollege/emailafriend.ascx" %>
<div class="blog" >
<h2><%# Eval("BlogPostTitle", true) %></h2>
<div class="bookmark">
<custom:EmailAFriend id="uscEmailAFriend" runat="server"
PageUrl='<%# UrlHelper.GetAbsoluteUrl(GetDocumentUrl())%>'
PageTitle='<%# Eval("BlogPostTitle") %>' />
</div>
</div>