If you want to add the volume, the calculation is as follows. Your current quantity of fermentables is 2.5 gal * 51 points = 127.5 gal*points. If you add 0.5 gal of water with no fermentables, you'll have (127.5 gal*points)/3 gal = 42.5 points. That is, a SG of 1.0425 (or about 1.043). If that's ok, go ahead.
If you really want to top up and don't want to sacrifice gravity, you could add enough DME or LME to the water to match the OG and add that instead. Just give it a 15 minute boil, cool it, and pour it in. I'd just let it ride though.