We have not really done any integration with sharepoint. But if the sharepoint infrastructure supports SAML2, then there are possibilities to get the integration done. But we will need information on sharepoint.
For the webapp to become part of the SSO, you would need to add some configuration files. I do not think any application code changes need to happen.
As mentioned here, http://www.jboss.org/picketlink/FAQ
we have had various users discuss how they integrated PL with various 3rd party security systems.
If you provide some information, then it will be easier to guide.