First, sharping levers will work on metal strings.
Most sharping levers work by flipping up a lever that raises a small bridge into contact with the string. This also raises the string slightly. However this requires that the original bridge be at a specific height. Otherwise the bridge may not done into a firm enough contact with the string, or if the original bridge is too low the lever can result in a displacement on the bridge.
Whether or not the commercially available levers will work on your psaltery depends on the bridge height and if the wood is strong enough away from the bridge where you have to install the levers.
Levers are available from Robinson Harp and Strings at http://www.robinsonharp.com
I have used these for years with good results