The easy way I have found to do this is to use the Auto-Silk Function that is under the menu Manufacture > Silkscreen. Open that menu and choose Reference Designator to be silk. Set the other classes and subclasses to none. Click the silkscreen button and a duplicate reference dez will get placed on the Manufacturing Subclass of autosilk_top.
You might want to turn on that class in the color dialog so you can see the completed results and make the color something like green so it stands out, additionaly you may need to turn off your refdez under the component class as the auto silk can put the refdez in the exact same spot as it is on your silkscreen - component class layer.
With the duplicate of the reference dez on the autosilk_top layer you can change it's size or move it to a different subclass if you wish. You can also delete the duplicates you dont need. When you generate your gerbers dont forget to include the autosilk_top in your output for your silkscreen.
Hope this helps