I am pretty sure Euroreef has a conversion kit (RS to RC) available on their website. However, I share the opinion of melev. I do my best to avoid any possible "critical failure" spots. A extermal skimmer overflow is always a pain and can be disasterous. I don't have to worry about comming home to find my sump water on the floor and my return pump running dry (at least not from the skimmer
Although a recirculating skimmer is a little more efficient, I have compensated by oversizing the skimmer. Other things to consider are initial cost - recircs. are generally more expensive, resale price - recircs. seem to hold value a little better, and you could always run your recirculating skimmer in or over your sump. I run a older equivalent of the RS 180 on aprox. 75 g system volume and love it.