When I hit the Source button it automatically inserts code form the entire page. It doesn't give me a chance to paste. My ultimate goal is to insert more than one source code i.e. youtube file on a page. I would like to duplicate the History page.

You can't edit the source code when using the "Source" view provided by your standard web browser. You have to use the Source button in the system:

1) Click Edit Site Pages
2) Click the Edit button for the page of your choice
3) Inside the editor window click the Source button in the upper left corner

Note most pages in the system can be edited by you, but there's a couple cases where the default content cannot be edited, and the History/Videos page is one of them. That content pulls from The People History web site and is copyrighted by them, and thus cannot be edited in any way. You can always deactivate the History page and create your own of course, you just can't edit the stock content there.