A LOT of stuff can be done using the .JSON files in your installation’s data directory. Professions can be created by looking at /data/json/professions.json . It’s very easy to add professions either to that file or through setting up a mod folder and making your own files. (I suggest the latter).
Adjusting the scenario would be /data/json/scenarios/scenarios.json . Also relatively easy, but you don’t actually have to do that for what you’re asking.
The Fled The Riots scenario allows all professions, so all you’d have to do is create a profession and it will automatically be available to use in that particular scenario. Other scenarios, though, limit available professions, so you’d have to add them to the list of possibilities.
