You can add on. The MAX RAM that I7 can have is 16GB and your going to add 8, so you will be fine. I caution, sometimes if you mix 2 different brands of RAM your PC may not work. If it does not boot up you then cant add your 8GB and you will need to get 16GB of RAM that is the same brand.
No guarantees when adding/mixing sets, would look for a full set of the amount you want/need. With 1156 would look at the GSkill NT series of value DRAM, it's among the few still available with the low density memory ICs (chips)
I replaced: Crucial 4x2gb Elpida PC3 8500u-7-10-80
with: PNY PC# 14900 which is 2x 4gb
Only ran the system for a few minutes, but all worked.....don't really know if that took care of the BSOD issue yet.
Would it be necessary to add 2 more 4gb sticks to maximize the system or add any of the other 2 from crucial.
Also my hardware tool from "Speccy" indicates that for the short term that I was using it that it was running at 664MHz as opposed to the 531 that the crucial was running at (if that even matters).