The short answer: no. The indirect transfers that exist fall well short of your "at a decent exchange rate" requirement (a requirement which IMHO is reasonable, given the number of points involved).

If you charge enough to have obtained those 400K MR points in a fairly short time, moving your charges to a credit card that earns AA miles (such as the Citibank card you just ordered) will help. Consider a Starwood card too. Since it's issued by AmEx, it's accepted everywhere you could earn MR points. You also get a 25 percent bonus on mileage transfers to AA if you transfer at least 20K points at a time, which shouldn't be a problem here, so you'd get 400K AA miles after $320K in charges instead of $400K. Plus, you get flexibility in using the points in case you decide that AA 1MM isn't your goal (or you reach it).