Unfortunately, this is not simple. If you have prepared you document using only basic LaTeX commands and simple math, you can try use our Latex2Wiki Conversion Tool. For more complicated documents we can recommend Pandoc, a universal document converter allowing to convert files from one markup format into another. In any case most probably you will need to edit resulting wiki markup.
Since updating to the new site, we have integrated Pandoc into our text filter system, so will soon allow you to be able to post LaTeX documents directly, though they will still need to perform some tweaks to perfect their appearance.