Merely changing tires to an 18" shouldn't have any effect on your camber, provided the overall rolling diameter is the same as before.

If your alignment shop is unable to get your camber to a setting that will result in minimal tire wear, then your best solution would be a set of CPP upper control arms which are adjustable. They will allow you to adjust your camber and caster back into spec and minimize any tire wear issues.<ul><li><a href="http://www.purems.com/products/product.php?II=996">CPP Control Arms</a></li></ul>