You are limited to what HTML is capable of. Unfortunately, HTML doesn't have the ability to do what you want. However...
When you copy from excel and paste into a text-only control (be it notepad or a HTML text area) the column values are separated by tabs and the rows by a carriage return/line feed. So you could provide a "paste area" where someone could paste text copied from Excel, then you just need to parse the text for the correct separater characters. In the case of .NET, look for Environment.Newline and the tab literal (" ", which probably won't show up with this is posted, but it's just a tab between quotes). Then you can construct whatever data structure you would like from that parsing.