Perhaps you need to look at DW Templates. See this article:
If you have re-usable bits of code you like to insert into all your default web pages, use the Snippets folder.
You CAN also edit the Default.html page located in your configuration > Document Types > New Documents folder once you set DW to the Doctype you want to edit.
You'll need Admin rights on your computer to overwrite the program file and won't be able to change it while DW is running IIRC.
That's the least destructive way I know of. If you pooch the file completely, DW should be able to generate a new one without reinstalling.
The code i intend to add is something like:
<!--[if lt IE 7 ]><html class="ie ie6" lang="en"> <![endif]-->
<!--[if IE 7 ]><html class="ie ie7" lang="en"> <![endif]-->
<!--[if IE 8 ]><html class="ie ie8" lang="en"> <![endif]-->
<!--[if (gte IE 9)|!(IE)]><!--><html lang="en"> <!--<![endif]-->
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">
<!--[if lt IE 9]><script src="http://html5shim.googlecode.com/svn/trunk/html5.js"></script><![endif]-->
What i expect is i can set my own HTML 5 DocType, so i can select this customize blank page & the additonal code i want is already insde there, so i no need to type manual everytime when create a blank page. Thx...
To make a Snippet...
Open your Snippet window by going to Window > Snippet. Open a document that has the code in it. Highlight the code in the Source View, then in the Snippets Window, click the + icon and give the Snippet a name. From then on, you can click an insertion point in your source and then click the Snippet name from the Snippet Window and it will insert the code.
There is also a file within the SYSTEM DW folder hierarchy that you can change by adding this code to it. Then ALL pages will contain that markup. BUT, there's no way to confine the markup to HTML5 doctypes. Snippets is the right way to do this....