If the eggs haven't pipped externally yet, there should be no problems from opening the incubator. If they have, then you can do this:
Get a squeezable disposable water bottle (one made from super thin plastic is fine--it doesn't have to be designed for squeezing) with the lid, and some aquarium tubing. Put a hole in the lid of the water bottle large enough to poke the tubing through. Put the tubing through and the lid on the bottle in such a fashion that the tubing reaches the bottom of the bottle. Then use modeling clay or hot glue to seal the joint between the lid and the tubing. Fill the bottle with warm water, and you can poke the tubing through a hole in the incubator (ventilation holes?) and squeeze the water into the reservoir without ever opening the incubator.
I keep one of these contraptions standing by during lockdown, because I usually do have to refill the reservoir at least once during lockdown.
Good luck!