I have 2 questions to ask on the webservice. 

1. Is there a possibility to define an Attribute in the webservice. I could find webreference are supporting the attribute. Means outsystem supports the attributes being a webrefernce but being a webserrvice it is not supporting it.

2. I could get the wsdl of webservice once it is defined and published through the url. Is it avilable to view or edit the wsdl with in the service studio ? .Whether we can we do a wsdl to code (top down approach) realization ?
In Service Studio out of the box, you can only define Actions, Input Parameters and Output Parameters.
You can also use the EnhancedWebReferences extension to add SOAP headers to your web service, but you can't direcly change the WSDL generated by Service Studio.