Unfortunately, it's not that simple. Even the possibility of controller support can have subtle but significant affects on game design.
One common one is party size - most controllers have four buttons and most RPGs offering controller support have a party size limit of four characters. Nice and convenient for controller users, but a limitation for those who want larger parties.
Another controller limitation on RPGs - inventory. If you have to use a D-pad or joystick to select items, you need to impose limits on inventory size to ensure anything can be accessed quickly. If you limit inventory size, you have to limit the number of usable objects in-game also. In the case of D:OS which is absolutely swarming with objects, this would be a significant change - as it would have been for Divine Divinity, the first RPG with object overload.
The most common restriction however is the inability to do precise selection with a controller. Mouse users can select items with near pixel-perfect accuracy whereas with a controller (or joystick) you'd struggle to get within 10 pixels. In FPS shooters, this can be compensated for through lazy-targeting (counting anything close to an object as a hit, rather than requiring an exact aim). In RTS games with large numbers of units the only way to handle this is to *not* have large numbers of units - either lower the numbers or have units auto-grouped and allow group selection only.
The effect of this on D:OS? Spell targeting for one - spells currently requiring exact selection of location (like creature summons) would have to be re-engineered to remove the need for precise selection (having any summons appear adjacent to the caster for example). Attack spells would have to be limited to "valid" targets only (like enemies in combat) removing tactical options like dropping a fireball on a nearby pool of oil. And object interaction (like moving chests) would have to be limited to scripted actions only available at certain points rather than the current anything-anywhere approach.
The problem is that there are many similar games that did this very effectively (Skyrim, DA:O), so it absolutely can be done in a way that does not take away from the experience.
So the addition of controller support is not a simple matter for D:OS and it could not be done without fundamental changes that would rob the game of its distinctiveness and individuality.
If the game's distinctiveness and individuality would be compromised by adding support for a peripheral that MANY gamers prefer, it has a lot more problems than anyone has realized.