Are there existing libraries and such to facilitate the import COLLADA files to Unity3d at run time?
I guess it is. But only if you write your own Collada interpreter. And you will not be able to save it in a native Unity format either, you will have to write your own serializer for that.
During runtime, all the Editor classes are not available to you for use, so: Yes, you are able to do so. But only if you do this yourself.
I wrote a .x parser and used www for the images for my runtime imports
Heres one I prepared earlier:-
Unity Runtime Import Library
Do be warned, I will be uploading a updated library in the next few weeks which has many fixes, tweaks and one/two really usefull extra features, cheers Jon