An API in what library? I suppose one could write a custom XmlReader (the Microsoft pull-model XML API) that would
process an Excel binary and could be plugged into a DOM builder to get
representation of the spreadsheet as if it were XML. I've never
heard of such a thing, and anyway the Right Thing is to move Excel's native
format to XML by default (which will happen in the next release, in case the
original poster wasn't aware of that).
For the time being easiest thing to do would be to load the spreadsheet
into Excel (or OO.o, or Corel, or
whatever other products support both Excel binary format and
XML) and save it as XML.