Can I add more cream to my thick and ready ganache?
Yes, you can try adding more cream in without hurting anything. However, when cooled it may not mix in very effectively, depending on how thick it already is. If you have trouble you just need to heat it up a bit and it will mix in no problem.
Of course, heating may be a better option than adding cream in making it more workable. If you add cream it will be easier to work with when cold but runnier, if you shape it when warm and it cools it stays where you put it better.