Settings -> Agents has a name format, don't know if that changes the default though.
You're right, can't seem to find a setting either.
I mean anywhere in the code:
https://github.com/osTicket/osTicket/search?q="from+name"&type=Code&utf8=✓
It sort of implies (based on the hint) that it should be in the department settings. So a checkbox for the department "Use department name as default from name" or similar would solve it, maybe make an issue on GitHub mate, someone might like an easy job for once. :-)