Can a Merkel 12 gauge sxs shotgun action be made into a double rifle?

YES, but the question you should have asked is, will it be a quality rifle afterwards. Again, YES, "IF" the action is in good shape, and if you don't chamber to high pressure cartridge in it, and the work is done properly!

Macd37 I was thinking of going with a 458 Lott.Where can a guy buy a Anson & Deeley action?

Elim, I'm sorry to burst your bubble, but no break action double is suitable for cartridges like the 458 LOTT. The pressures are simply too high, and the rifle would shoot off face very quickly, if not on the first shot. The LOTT is a very good cartridge, in a Ruger No1, or a good CRF Bolt rifle, but is simply too hot for a double rifle, whether O/U, or S/S! The double rifle is, eventhough absolutely brand new,still 18th century technology, and must be used with cartridges produceing no more than 40,000 psi chamber pressure, and are far more suitable with a FLANGED, cartridge, that produces pressures in the 28 to 30,ooo psi range. About the highest pressure cartridge that is really suitable for a double is the 375 H&H Flanged cartridge with loads that go no higher, in chamber pressure, than factory ammo. Though the factories sucumb to pressure from customers, and chamber cartridges like 375 H&H belted mag, and 458 Win Mag, this is a mistake by the customer, and factory alike, IMO! A double rifle is not the place to try for more speed. They were not made for that purpose. The double rifle is the only rifle in existance, that was designed strictly for "HUNTING", and nothing more. It works best with low pressure, and flanged cartridges that push big heavy bullets at just enough speed to "KILL" big animals, and maintain rock solid reliability, at the same time! A 450#2 NE with a 480 gr bullet, running at 2150 fps will kill anything that walks the Earth, and do it with style,while generateing no more that 28,000 psi chamber pressure! If you want a classic rifle in a cartridge like the 458 LOTT, I'd go with a Ruger No1 so chambered. This is an easy conversion, and will certainly handle anything the LOTT cartridge can dish out!

As far as buying a new A&D action, I'm not sure they are available to the public.