Is there a way to add new feature to existing machine learning model and retrain it?
So the question asks if you can retrain the model after adding another feature.
Answer is yes but you need to instantiate a new model which can accept a 3-feature/dimensional input.
This is due to the mechanics of neural networks whereby in the 1st layer, the features in your original model would be multiplied by a ($2 x n$) matrix where $n$ represents the number of hidden neurons in the second layer of the model.
Therefore by multiplying a ($3 x 1$) input with the weight matrix will throw an error.