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It wouldn't be difficult at all. Your site almost looks like a XOOPS site as is.
IMHO, it's best to base your theme on the XOOPS default because you know for sure it will work if you follow standards and don't get crazy with too many tables.
To convert the default theme to look like your site, ad a table row for the navigation bar below the header, insert your javascript navigation bar, edit your CSS to match colors, fonts, and table widths, and manage your left and right blocks using the XOOPS Blocks administration.
If you're comfortable editing HTML and CSS, it would probably only take a day or so to get it done.
My advise, setup a local development server with XSAS or easyphp, and start hackin the XOOPS default theme.html and style.css in a controlled environment.