Is there any tool could define/modify the .xsy file?

Yes, there is an excel tool is published with Unity v10, it can define/modify the exported .xsy file in excel. (the path of the tool is in Unity installation folder -> \Extras\Excel Import Export Tool)
Note: it only support with Unity v10 or later.
Please see the attached pdf with the intruduction of the tool.

Released for: Schneider Electric USA