How to add negative data ?

The reason i am asking this question is because of the algorithm implemented here works differently for negative data.

For example, Faster R-CNN considers as negative boxes those which have IoU under a certain threshold (0.3 by default, I think). And then it takes as many negative examples as positive ones.

So basically, if you have your object B in images with A as well, and it is not labeled, it is some chance that it will be considered as a negative.

Please correct me if i am wrong !
Can you tell me how can i add negative data without tagging ?