Depends on how often you visit the hive. They have used only 1/3 of the sugar since early December. So you could probably wait another month at least before adding more sugar. But if you wanted to add more now it would be easy. Just lay a folded piece of paper on top of where the bee cluster is, overlapping slightly the rest of the newspaper and add your sugar. Spritz some water on the sugar so it will clump and not fall between frames.
Not sure why you were reluctant to add more. If you lay paper on the bees, most of them will move down or out. You might crush a few but that's the price of feeding the hive.