Cédric, as I explained in a previous post, you can consult the SOAP that Efficy Sidebar and Add-ins send to the server.
Opening these files on the client:
In this case, the Field selection dialog executes the gettemplatefields operation.
In returns a huge list of pipe separated fields, too large to include in this reply.
The content will be dynamically populated based on the meta data in
SYS_TABLES. I do not believe there is anything you can modify. If you do not have your field available (for instance a virtual or calculated field), a template script could be your solution.