But this guy from this thread ( http://www.bay12forums.com/smf/index.php?topic=164680.0 ) managed to make a dfhack hack that can cause sieges by taking control of moving armies or something. And in the third post of this thread ( http://www.bay12forums.com/smf/index.php?topic=161765.0 ) a guy says that there is files for when a civ sends a siege that could be modified.
Yes, siege triggers. That's not "forcing a siege" as Dfhack may or may not be able to do (no idea, it was you who said it couldn't be done).
https://dwarffortresswiki.org/index.php/DF2014:Entity_tokenJust edit raw\objects\entity_default.txt with a text editor and change any of the options you want (guide to what you can set things to in the above link).
Goblin settings start at [ENTITY:EVIL].
You can set sieges to come according to population number (default is 80, way too high in my opinion) and wealth levels
(switched off by default, I find it more fun if they're used) for each civ individually.
For a start PROGRESS_TRIGGER_POP_SIEGE for goblins is set to 3 (population 80). Change it to 1 or 2 (population 20 and 50 respectively).
Or raid someone. That overrides the triggers.
None of the above raw changes will "force" a siege though. If there's no goblins in your world, or they're busy being killed by everyone else, they won't come. Embark close to goblin and necromancer sites for the highest chance of Fun.
(Necromancers don't care about your population or wealth, they'll come when they feel like it, sometimes much earlier than you'd like).
(Or use Dfhack as the people in your links say. If it actually works, it's likely to make your game highly unstable).