I recall seeing a fresnel attachment/accessory made for studio strobes, but don't recall the manufacturer. Check Calumet or B&H and you may turn up something. As I recall, however, it wasn't particularly inexpensive.
I think Midwest Photo in Columbus, Ohio had one on eBay in 2004. Maybe someone there would have more info.
BTW, I've been looking for one of these too. I've been thinking of buying a cheap PAR spotlight from a local theatre lighting house and converting it to work with my Alien Bees.