How can I adjust wheelbase?

All of this with the alignment is true. You can certainly influence the wheelbase with alignment adjustments. But the original question was "Is the wheelbase adjustable?" The correct answer is, no it is not. There is no factory adjustment for wheelbase, because it is meant to be a fixed measurement. As in when all factory alignment specs are correct, the wheelbase will fall into spec. If you're having to try to adjust wheelbase using alignment adjustments, then you most likely "have something wrong".