How we can get the list with all physical units and prefixes from WolframAlpha and make symbols from the result to be equal their corresponding quantities like described in this post?
I'm not sure whether this is what you seek, but you can use
Trace to investigate in a call to
Quantity. Then you extract the essence
Quantity["Newtons"]; StringReplace[Names["CalculateUnits`UnitCommonSymbols`*"], "CalculateUnits`UnitCommonSymbols`" ~~ r_ :> r]
and you get some kind of list ;-)