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Usually we get questions asking how to integrate their site into xoops, and not the other way around as you have posted your question. As in, how do I integrate allready created HTML pages, so that they appear as my XOOPS site looks.
You will need to explain a little further, what it is exactly what you want to achieve. From what I understood:
If you want to have sidebars, as you have them on your current site, you can create custom blocks, and place them where you want in the block settings.
If with header you mean logo and menubar, this would be a change to the theme you are using. Just as well as the overall look/design of your site, is governed by your selected theme.
If you are concerned about meta tags, see the following.
In Sys Admin -> Preferences -> Meta Tags and Footer, you can put what you want to appear in your footer. Though note, some themes wont display the footer, as they don't include the smarty tag for placing it.
Hope this gives you a start...