Sure, as long as you don’t contaminate the brew in the process. Key to keeping things sanitary is to minimize the amount of time you leave the fermenter open. During fermentation, CO2 will move out of the fermenter when you open it and this will repel outside contaminants. But this process will only last for a short while so you can’t just leave the fermenter open while you go eat lunch. Also, you need to be sure that anything you bring into contact with the fermenting brew has been thoroughly sanitized.\r