Capturing an HTTP Body and adding it to SAML attribute

We have a requirement to capture a portion of an incoming HTTP Body and add it to an outgoing SAML response. I also need to convert this to XML after capturing. Once that is completed it will need to be added to a session variable and sent out in a SAML response. 

Is this the best way to do it? This will involve using a combination of iRules and APM policies.

Any guidance, examples and documentation on this will be appreaciated.

Thank you