Once you decide to bentch (however you define that: thinking it, washing mayim achronim, pouring a kos shel bracha, starting zimmun, and for sure according to everyone if you actually started bentching!), you would have to make a new bracha anyway on any more food* or drink that you decide to eat* or drink (Shulchan Aruch OC 179:1). So there is no reason to stop bentching as even if you did, you would still already have to make a new bracha rishona.
*The Shulchan Aruch brings two opinions if food is similar to drink in this regard. According to the Mishna Berura the main opinion (that of Rosh) is that they are similar, but it is proper to be stringent and avoid eating anything once one has decided to bentch due to the doubt related to the bracha rishona on that food.