Insert (reorder) your existing holes into a new partbody, and then add the mirror command to the "current solid"
I agree that it seems a little awkward at first, but I do this all the time for mirrored, rotated and patterned features.
Based on the image of your part, here's how I model a set features: Make a sketch of the center points on the right side and mirror the points about the center plane so the sketch contains all the left & right centerpoints. Make a Hole at one of the points, and do a User Pattern to copy the hole to all the other centerpoints in your sketch. I'd do this within a partbody, but it's not necessary if you select the first hole before you click the User Pattern.