Hi Sherwin,
My question is can we create xsd in sap pi?
No you can´t do it directly. You should create the data types and message type manually and from message type you will have a tab with the XSD.
if we cant, is there any tool which can convert wsdl to xsd? if yes which tool and how?
You can use the free tool SOAPui tool for this purpose. (http://stackoverflow.com/questions/23238785/how-to-generate-xsd-from-wsdl)
XML spy can work how?
I used the XMLSPY for create XSD from XML example, but i haven't so clear that it will work for wsdl. On the other hand i advice you to use free tools to work with XMLs like EditiX.
Regards.