Using IVR to Add a SIP Header

Hey all,

I'm working with a product called Jitsi to host some video conferencing for my company. To setup the dial in number, I need to have the users call into my phone system (FusionPBX), go to an IVR and enter a pin number (won't always be the same), and then add a SIP Header containing the pin entered and send the call off to my Jitsi box. How do you add a SIP Header to a call with FusionPBX/Freeswitch? Also, how would I use an IVR to accept any input and store it for later use in the SIP Header?

Thanks in advance!