CNN on python with Keras
What you want to do is called "transfer learning" using the learned weights of a net to solve a new problem.
Please note that this is very hard and acts under many constraints i.e. using a CNN that can detect cars to detect trucks is simpler than using a CNN trained to detect people to also detect cats.
In any case you would use your pre-trained model, load the weights and continue to train it with new data and examples.
Whether this is faster or indeed even better than simply training a new model on all desired classes depends on the actual implementation and problem.
Tl:Dr
Transfer learning is hard! Unless you know what you are doing or have a specific reason, just train a new model on all classes.
I made a simple CNN that classifies dogs and cats and I want this CNN to detect images that aren't cats or dogs and this must be a third different class. How to implement this? should I use R-CNN or something else?
P.S I use Keras for CNN