Hey Denis! The Tank unit can be Refit with any Western Allies PP's, which will almost always be French or British in origin.
The US could use the Tank benefit if they share an area with the Tank unit and it is Fresh at the time the US attacks. Normal rules apply and the Tank would then become Spent (and thus not useable by the Western Allies the next turn).
The U.S. can't actually move them (as they are WA units) but the special rule allows them to be used if they are Fresh and in the same area with the U.S. units. The U.S. depended on French training and equipment when they first arrived in Europe.