Need to post an extra parameter along with samlResponse to SP

[Nguyen, Thai]

Hello,

I have an IdP working with a few SPs.
I am current stuck with a new SP that requires to post an extra parameter along with samlResponse.
This SP is using IdP unsolicited SSO feature. I had configured so that when user click:
https://myidp.example.org/idp/SAML2/Unsolicitied/SSO?providerId=https://sp.external.org
My IdP generated samlResponse and post it to SP’s ACS url correctly.
However, the mentioned SP required that an extra parameter need to post with samlResponse.
Ex: https://sp.external.org/SSO?samlResponse=Base64EncodedSamlResponse&extraParameter=some static value

Is it possible to configure IdP to add the extra parameter?
Please share some guidances. 

Thanks in advance,

Nguyen, Thai
tqnguyen at usfca.edu