you can but... I would not add anything that you don't test for... so test you alkalinty first (aka kh, carbonate hardness, etc... ).... alkalinity and calcium are closely related so its best to test for both if you supplement for either...
Are you sure your pH readings are correct???
it is possible to have low pH and high alkalinity readings-- the CO2 in your water can affect your pH even if your buffer concentration (aka alkalinity) is real high.... if this is the case then adding that won't help your pH...
Read the link I posted above... it is very informational concerning this subject... I can't begin to give you the advice Dr. Farley can... you can also check out the chemistry section here at RC...