Is there any program that can help you mod BG1+2? Such as Aurora Toolset is for NWN and Source SDK is for Half-Life 2?

You need third party, fan made software to mod the IE games.

It's no small task to mod BGII. Making new maps is a very complex process, making new encounters on current maps slightly less so. Adding quests is major work.

The Aurora Toolset is childs play in comparison to what modders use for BG.

I've never modded for NWN and therefore do not know the Aurora Toolset at all but you make it sound as if modding for IE-games is quantum physics. Which it is not. The only real pain is making new areas (and I mean new and not just reused ones) and even that is mostly the 2D rendering of the actual picture you walk on. But everything else? New quests, new party-NPCs, new items, new rulesets, new stores and whatever else is not that hard.

Although you have to code stuff directly (but in a very basic way). That might be where the childsplay ends, I guess. As I said I never even looked at the Aurora Toolset so I don't know how one would go about scripting stuff there e.g.