Showing header and footer of oone Blog into another Blog

Mi problem is:

I have this Blog: https://severus.es

It is a multiblog platform WP 3.0.1 that order the Blogs like this: https://es.severus.es, https://com.severus.es, etc.

Now, I would like to add this shop based in another WP theme: https://com.severus.es

My problem is to visually integrate the header and the footer of the main site (https://severus.es).

To switch off the header and footer is easy, the difficult point is how to make the functions get_header and php get_footer to call the header and the footer of the main site and show then in the “child site” with the style keeping the style of the main site.

Any idea?