You can add any field you like:
sip_custom_post.conf
This is the file that allows you to add/remove values to those entries found in the auto-generated sip_additional.conf file. So for example you have an extension 1000 that needs an additional parameter added. Create a context line: 1000 then on the next line add the item(s) you need to add. To remove use (-) instead followed by the line(s) you want removed.