Smartsheet utilizes the ability of the Google API to import an Excel file. The code is roughly along these lines:
DocsService client = new DocsService(<YOUR APP NAME>);
client.setOAuthCredentials(<OAUTH PARAMETERS>);
DocumentListEntry newEntry = new SpreadsheetEntry();
newEntry.setMediaSource(new MediaByteArraySource(<EXCEL FILE BYTE ARRAY OUTPUT STREAM>, DocumentListEntry.MediaType.XLS.getMimeType()));
newEntry.setTitle(new PlainTextConstruct(<FILE NAME>));
DocumentListEntry insertedEntry = client.insert(new URL("https://docs.google.com/feeds/default/private/full/"), newEntry);
// This is your URL to the new doc
String docUrl = insertedEntry.getDocumentLink().getHref();
We already had the ability to export a Smartsheet to an Excel file with formatting via Apache POI. Adding export to a Google Spreadsheet was quite simple for us to implement and it provided some additional functionality beyond what you could do via the API.
Sorry for the delayed response - just happened across this question.