I have the same question...

It isn't implemented... aka, mod is sort of busted there.

As the starks i think your only option for expansion is to attack the Nights Watch, and then attack the Free Folk. unless you force the boltons to revolt, then maybe you can eat them? i noticed unless youre a great house bordering another great houses vassal you cant really do much. try house tully and you can fight the lannisters, maybe dorne against the storm lands, but aside from those i dont think war against another house is optional. you could play as the crownlands for more combat i suppose. or play on essos. but your odds of justifying a war and winning against the 7 kingdoms is slim next to none. You could also develop your lands until you feel confident enough to war the crownlands? maybe try the Arryn, Tully, Stark alliance but the reach- dorne- lannisters, and the stormlands will defend the crown. so a civil war is realllly hard to win.

also if you play as a minor house you can take other minor houses lands in lordly disputes- from there you can challenge your lord for the Warden title.

just integrate a feudatory that has access per land or water to a neighbouring region like the riverlands, vale or iron islands. the nobility estate give you the option to integrate your feudatories. After expanding your land like that you can fabricate claims like always and gain cbs