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It is not that hard to change a good layout to an XOOPS theme. If it should look exactly like the existing layout and has sortof the same general layout (header at the top, blocks right left and center) it is very easy.
E.g. I work off templates from TemplateMonster and count 2 hours of work to turn it into an XOOPS theme.
A lot can be done simply by changing the default CSS and theme.html to look like an example page from the existing layout.
Of course, if most of the existing layout is done directly in the pages and not through CSS, it is a bit harder.
I'd recommend that you copy the default theme and change the style.css by copy-pasting bits from the existing layout's css file (provided there is one) as well as copy-paste the header part from the existing HTML pages into theme.html, replacing the things already there.