Regardless of which SIP driver is in use, the best way would be to use the native FreePBX subroutines for adding headers. You pass two arguments to the subroutine, the fist is the name of the header, the second is the value, separated by a comma:
exten => s,n,GoSub(func-set-sipheader,s,1(X-Account-Code,90210))
I don’t recommend you add _1xx as an exten to from-internal-custom, I suggest you review this post about dialplan hooks: Hooking for fun and income