This is a strange question because it depends on what is meant by "interfere" with. The addition of water will NOT interfere with the determination of the molarity of the sample being analyzed, but the molarity of this sample will most certainly be less than the molarity of the original sample.
If the molarity of the diluted HCl samples turns out to be, say 0.5 M, that would be a correct value as the water did not "interfere" with the determination. Now, is that the molarity of the original sample before you added the 25.0 ml? NO, of course not. The molarity of that original sample will be greater than 0.5 M because you diluted it with water. It would be 1.33 M in the original sample (25 ml + 15 ml = 40 mls and 40/15 = 2.67 fold dilution, so 2.67 x 0.5 M = 1.33 M).
Did the water interfere with the determination of molarity? NO. Did it alter the molarity of the sample? YES.