Add a sentence to an extension configured with freePBX

I want to add the following sentence to the configuration of an extension configured with freePBX, but I want it to be persistent, no matter if I upgrade or change the configuration of the extension in freePBX:

language=es

What I did:

sip_additional.conf:

[201]
(freePBX configuration)…

sip_custom.conf

201
language=es

Since the order of loading this additional files is first custom and then additional, asterisk wont start. Any comments on how can accomplish this addition?

I brought this question from the Trixbox.org forum
http://www.trixbox.org/modules/newbb/viewtopic.php?topic_id=4421&forum=1

bump