I added a fan to the stock cooler with manual activation, not quite the set up you were asking about. I only turn it on very rarely as my trans doesn't get past the 170* I use as my limit to turn it on very often.
I really wanted to do it. The more I looked at it though, that fan is to far back. I am sure it works to some level, I am not saying it's useless, but I have this cooler (http://bmracing.com/?wpsc-product=h...ng-system-13-12x9x3-12-with-9-12-diameter-fan) and with the shroud around the fan ensuring that the only air it pulls comes directly through the cooler, the results are extremely fast when trying to cool the ATF.