Only way I know how to do this, albeit you should absolutely undo this before publishing a mod due to cross-compatibility purposes, is the following:
Copy the "Start" goal to your mod and either just edit step two, or add a new origin entry in the INIT if you have a custom global character you wish to use.
Change line 151/152 to specify a specific integer rather than the random value, so it always spawns that entry in the DB_Origins database.
You probably could do some clever workarounds to just override the conclusion of 'start', but since this is an engine-thing only I don't see a reason why you'd ever want that so I won't bother chewing on that puzzle.
If you have an actual new Origin you want to add, you should follow the "My first origin" guide on the wiki, though it's down for maintenance at the moment so I can't grab the link for you.
The TL;DR of it is basically for the story bit, you need to add a new DB_Origins entry through a new goal placed under DOS2ModWrapper.