The question shouldn't be can threejs be used in C2, but instead could C2 be used to create 3D games.
Let's tackle the second question first as it's already been commented on.
"could C2 be used to create 3D games"
The answer is yes. In fact some one already did this with Copperlicht engine. It's not a monumental feet not insanely difficult. However doing so would pretty much make everything else useless. You can't use Sprites and all sorts of yummy objects. Why becase they would be rendered under or over the 3d rendered world. There is no integration of objects. So such other objects would be limited to that of the UI.
If your doing this model then the collision detection would be based on what 3js already provide and so no need to create any aditional work.
Let's tackle the first question second because it's the better way of using any 3d api.
"can threejs be used in C2"
The answer is YES and without much difficulty, but doing so will require re-thinking and working around the situation. Instead of 3D worlds, think of instead 3D objects. ThreeJS supports texture based rendering. This enables the dev to render an object/group to JS Image. This image then can be used as a sprite render replacement.
If you go with this design. Then your using C2 collision detection and no need for additional work.
The answer is overall. yes to both, but it's only really worth the effort to create as an object rather than a 3D world. It's not difficult just time consuming for the effort and research. I however may be looking into this myself shortly. I also did the research ahead of time to know this stuff.
Now however if you want to actually do C2 as a 3D game engine. That is still doable. However this would indeed required making many new Plugins as replacement's to C2 already fantastic Plugins. All Behaviours relating to position would be need to be made to have 3D replacements. Then you would need an object in the IDE to act as camera control. It's doable, but would take time.Probably a few months.