I have heard this, too. It seems agreed upon that there's a common root, but there's some question as to how exactly that came to be: http://www.randomhouse.com/wotd/index.pperl?date=20010214
"Ademas" means more like "furthermore" or "in addition to". "However" has a more negative meaning and means more like "despite that" or "on the other hand," which you should use the word "pero" or "aunque" instead of "ademas." It is hard to explain but I hope you and others understand what I am trying to say.
Why can't we say "what's more"? It's the same as "besides" and "furthermore"