No, you cannot use ASP.NET controls inside the html content.
In ASP.NET WebForms there can be one and only one form on a page. If you want to do something with a form submission, you can easily implement a custom feature.
To make a default button in ASP.NET wrap the form controls inside a Panel and set the id of the buttonas the DefaultButton property on the panel. If the users hits the enter key while focus is on any control in the panel it will fire the button click.
<asp:Panel id="pnl" runat="server" DefaultButton="MyButton">
<asp:Button id="MyButton" runat="server" Text="Click Me" />
Hope that helps,