I found this thread:
http://forums.contribs.org/index.php?topic=38614.0, but it's eight years old and two major releases out of date, and I don't see that it actually answers the question (it says, "don't do that" instead). Following from that, it looks like there's been a bug open requesting this for about 10 years (bug 319, which has been superseded by bug 2669, which itself has been open nearly nine years), but the feature doesn't appear to have been implemented.
I did search the forums (specifically, I googled "add network interface site:contribs.org", which includes the forums), and got several hits, but nothing describing how to implement this, in the French section or elsewhere. Was there a particular thread you had in mind?
For my purposes, Daniel B's suggestion should work--it's simple, it doesn't affect any configuration files, and it goes away when I reboot the server. But if there's something else documented, it could be useful to someone else who might stumble on this thread.