Good news: the apostrophe-pieces-import module now supports Excel files natively. No need to convert to CSV first. This is very helpful because Excel does not convert the character set to UTF8 properly when exporting CSV. Native import of Excel avoids this problem.
Also, there is a simplified interface for adding your own formats. The stream-based interface is still available but there is also a simple callback-driven interface for formats that don’t easily support streaming.