You will likely be waiting forever, , they were removed from Steam years ago.
This mod is the only way to get them, for both GOG, and Steam users

Licencing is a mess
Microsoft Game Studios Published DS1, and 2 made by Gas Powered Games

DS1 Expansion Dungeon Siege: Legends of Aranna was developed by Mad Doc Software, published by Microsoft Game Studios

DS2 Expansion Dungeon Siege II: Broken World was developed by Gas Powered Games, but published by 2K Games

Then Gas Powered Games sold the Dungeon Siege Franchise to Square Enix, and rights, which included the now lapsed Publishing Rights, of Microsoft Game Studios,

Dungeon Seige 3 was then published by Square Enix, and developed by Obsidian Entertainment
DS1, and DS2, were removed from Steam when MS right to publish them, had lapsed, but both base games were then republished on Steam, and now GOG by Square Enix.

However, Gas Powered Games, and thus Square Enix never owned the Rights to either expansion.
Gas Powered Games do not own rights to Legends of Aranna expansion, they did not develop it, Mad Doc Software did
Microsoft Game Studios did not hold the publishing rights to Broken World expansion they did not publish it. 2K Games did.

It's a mess of conflicting ownerdship, and publishing rights that means both Expansions are unlikely to be added to either site.
If anybody can wade through that mess, it will be GOG, but it may never get past the roadblocks.