I have not been able to find any NAML code that generates the From: address and fear it may be in the JavaSript or other code behind the software that cannot be edited by a user.
I find I have to warn users to look for mail from a nabble.com address, which sometimes does find it's way to folk's spam folder. :-(
I would love to have the option you request too. I suspect it would require getting your forum setup as a subdomain under your client's domain and a tweak by Nabble, as they have to to cope with https.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.