To change the ownership of settlements, edit descr_strat.txt

There is a good thread in the guides section for editing the campaign maps, check that out...

Rebel regions do not need to be defined in descr_strat.txt so to do what you suggested, simply delete all the settlements you want to be owned by the rebels from the germania section... That wasn't very clear sorry, here we are:

This is the section of code for the settlement of Damme (Tribus Chattii):

Code:
settlement
{
	level large_town
	region Tribus_Chattii

	year_founded 0
	population 3000
	settlement_tax 51
	plan_set default_set
	faction_creator gauls
	building
	{
		type core_building governors_villa
	}
	building
	{
		type defenses wooden_pallisade
	}
	building
	{
		type barracks muster_field
	}
	building
	{
		type missiles practice_field
	}
}
If you deleted this section of code, the settlement automatically would belong to the rebels.

However, you also need to delete (or move) the garrison that would be in Damme:

Code:
character	Arminius, named character, leader, command 0, influence 0, management 0, subterfuge 0, age 60, , x 95, y 130
traits GoodCommander 2, Intelligent 2, PublicFaith 1
ancillaries bodyguard, civilized_slave

army
unit		barb chieftain cavalry early german				exp 1 armour 0 weapon_lvl 0
unit		barb cavalry german				exp 2 armour 0 weapon_lvl 0
unit		warband spear german				exp 1 armour 0 weapon_lvl 0
unit		warband spear german				exp 1 armour 0 weapon_lvl 0
unit		warband spear german				exp 1 armour 0 weapon_lvl 0
unit		barb screeching women german				exp 0 armour 0 weapon_lvl 0
If you change the x and y coordinates to
Code:
x 97, y 132
, the garrison will be next to the town instead of in it. Deleting family members can cause problems because of the family tree. Hope this helps.