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This doesn't really answer your question directly, but you could always show block data along with your default module to achieve something close to the same effect. For example, you could enable the Recent News Topics block and place it top/center while making your forum the default module.
Your users would then see recent news articles at the top of the page and the forum just below that. If no blocks exist that provide the exact look you want, you can always create a custom news block that shows exactly what you want it to.
This is all pretty basic XOOPS stuff -- sorry if I'm insulting your intelligence.