Modify SAML request

Dear all:
I need to know if it’s possible to modify the saml request sent to one IdP Provider, I need to add, for example, Provider Name, or X509 Certificate. Can you tell me if it’s possible and how?

Please, can you tell me if the request is created with one xsd scheme? If so, can you tell me which xsd scheme and where is it?

Thanks in advance.