You can Paste the HTML source code on aspx page with all css,image folders (if html page have) and change the HTML controls to ASP.Net Controls manually.
For Example:
for Form tag in Asp.net ,You can change as
<form runat="server">
by adding runat="server".
and some controls can be changed as
<button id="id" text="label" önclick="sub"> in HTML.
<asp:Button id="id" text="label" OnClick="sub" runat="server" />
in asp.net by adding ASP and runat="server"</button>
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