shibboleth SP - how to customize HTTP redirect request?

[Paolo Veronesi]

Hi,
i manage a shibboleth SP (shibboleth-2.6.0-2.1.x86_64) on Centos7.X with 
apache 2.4.

A vhost is protected by shibd and it is configured to reach a gateway 
that act as IDP with my SP and as SP with lots of other IdPs, and it works.

Here an example of HTTP redirect request sent by my SP to the gateway 
(https://fakegateway.com)

https://fakegateway.com/gw/SSOProxy/SAML2?SAMLRequest=zZTBbtswDIbvewpD91i226WJEAfIHAwL0K1GnO2wSyHLdCPAljxRXtK3n%2BxkaQokLrZTfdBBIn%2BSH0nPkNdVwxat3ao1%2FGoBrbevK4Wsf4hJaxTTHCUyxWtAZgXLFl%2FvWeQHrDHaaqEr4i0QwVipVaIVtjWYDMxvKeD7%2Bj4mW2sbZJRaJ45N5Iu2iYIg8KWl2MiCZluZ57oCu%2FURNe3kI5o%2BZBviLZ2LVLxTftEpoQDD%2FQqcM%2B9UnnY0yx5So%2FfPB2%2FirZYxeRyLAop8clvwkI8n4Q3P4TYaC3fefZyWITgzxBZWCi1XNiZREI5Hwd0omm6CKYtCdjP5Sbz0WOUnqQqpnoaR5AcjZF82m3R0KOIHGOwLcAZk%2FsG79M063KxPx5w1YDgY%2F0udzN9ijCfGM3oWaiibhn1zEVfLVFdSPHuLqtK7xAC3EJOSV%2Bjwfdam5nY4ye5GFqOyN2XWcIUSlCV0OPZxGKHoR9ONlYW99RJdN9xI7GjCngt7jecJ6bl7Ujliayj%2FB%2FCgmWCik3bXGYjWwBpqbSF1VzttiiPxS5kMTsM7T%2F3wdqVTlwo7djYTunEr4vX7mui237y3%2BvgSxqyW%2Fzztr72vMKev0juVd%2F5rnP8B&RelayState=cookie%3A1469784098_a91d&SigAlg=http%3A%2F%2Fwww.w3.org%2F2000%2F09%2Fxmldsig%23rsa-sha1&Signature=OAGLz1qJpuPAYXLPggjPfvABXb5EG5C6QC0mL2ljtsKHrDdXM4y4bFWns2%2FcfibKJRMgdsWQPPS8sNflIWDPnY088fCsITSWRipImwPAQnijkRLPbaHZBOfJutpnm%2BnJLDEhNCR8hf5lKhzgumFirlVRs66UZQ32qa1h1X3QUWfg74g8njOF4xcUACGRtqxlrePlYQi%2BCftgmW8UpDpHFh%2F5vfJ7hIHu1Q9uBWIZ4V25D8GejCMNU%2F8a50%2Fp7rHtiPHJv08XYhkLPq9HgbG2CtZNL%2F3FjJf0aMHl8%2FVEP6KGAWMJMre1f4D%2FY8%2FK6VADy1J7kBHhmmLyy4EJdGJodA%3D%3D

Is it possible to change the HTTP redirect request created by the SP in 
order to append a custom attribute (customAttr=value) with the value 
selected in a form by a user?
This attribute should be used by the gateway to skip the default idp's 
chooser page and redirect the user to the appropriate IdP's login page.

Kind regards,
    Paolo