you should be fine adding lemon extract to you plant to add flavor. they recommend adding any extract 1-3 hrs before harvest. because of the extra sugars, they can rot your plant if you add them a long time before harvesting. due to the time period you add them before you harvest them it should not be a problem with throwing off the PH.
there is a guide in the harvesting and processing section of the forums.
http://forum.grasscity.com/harvestin...g-flavors.html
hopefully this should help. sorry if it did not.