Tensorflow Object Detection API
To build a model to detect the old categories and your new ones, you need to re-train the model with your own dataset and the dataset used to pre-train the model. Fortunately, this dataset is available to the public for downloading. The dataset used to train the model is written in the name of model you choose. It's probably the COCO dataset.

Since the model as been pre-trained to detect the old categories, I think you don't need to download all the dataset. The data is just necessary to avoid the model to "forget" what it has learned.

I hope you understood me, and that I have answered well your question.