Well, the trouble is that Mongolian culture (if im not mistaken) is based around the steppe, and the steppe pretty much ends on the borders of the Mongol empire. The second disadvantage for the Mongols is that there are no organised societys for them to conquer. So I suppose the logical way to open up Siberia for Mongol settlement is the get rid of those trees, or have a Siberian civilisation beforehand. Otherwise, I wouldn't see how it is possible.