I think its already possible with the labels overhaul (and partially even before)
Anyway, the game already does that, joining the rebels is regular and you can already make it more frequent by modifying a line or two in descr_campaign_db. Similarily, it is already possible for generals to join other faction, be it a bribe or shadow faction mechanics (barons alliance in britannia DLC), so it might not be that difficult to do what you suggest.
Neverthless, we should be already able to do it for most part, using this version's labels overhaul. As you now can do almost whatever you want with all generals at any point of the game. So if there is some general who would be supposed to rebel/start civil war or something, based for example on his trait, event, loyalty or something like that, you can choose him in the script, and for example give him another (hidden) trait that would decrease his loyalty tremendously, thus he would definitely join rebels till the next turn, or a shadow faction if the faction has any. And I believe if he was inside a settlement, the settlement would rebel too, right? Or would he be kicked out? Anybody knows?
Though,there is a "provoke_rebellion" command that is supposed to force settlements rebel, but based on this
discussion it doesnt work. But there is another command, "add_settlement_turmoil settlementname value" that works. It increases unrest, but you need high values to make the settlement rebel.
You can also use "barons alliance" emergence script for any faction, assuming the shadow faction in the script are rebels(=slaves. Rebels are de facto shadow faction to all factions so it works), so with doing some smart scripting you can do it to force a settlement of your choosing to rebel. I recall there was a guide on this, but cant find it RN. Anyway, the basics is that shadow factions emergence check public order in each setttlement and the ones in a public order limit you make can rebel, and you can also set how many settlements should the original factions keep, so through this there is a way. You could also combine it with "add_settlement_turmoul" to change public order to what you need. With labels now you could also make sure the govenor becomes "bad" to force it.
But yea, if there was a simple commnand for it, I suppose it wouldnt hurt