how to convert ".html" pages into asp.net ".aspx" pages

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>

You need to redesign and remake your html page in aspx.
AFAIK, there is no other way.

There may be a few tools to help you out, but I dont expect any of them to work completely.