adding new symbols into the electrical library

Is it possible to add new electrical symbols into the electrical library?

yeap, easy breezy.

One way is to explode an electrical symbol and change it or add lines etc.  Then go to System Libraries, then on the type change or put to "electrical" then pick a category and do "One step" and put a box around ne symbol, and it adds.

You can draw up anything and do this with out exploding anything.

I want to bring in 3D symbols from other sources into my library. Can that be done?

Yep. Save to the 3D symbols folder to a folder of your choice.