You’re using SipAddHeader which only supports chan_sip. You want to leverage the built in subroutines for adding headers, they’re channel driver agnostic:
exten => 100,n,GoSub(func-set-sipheader,s,1(Jitsi-Conference-Room,${JITSI}))
Once you confirm the above works, then you want to eliminate using the override file, and instead use a hook: Hooking for fun and income