I assume your DJ is already filled to the shoulder, it will probably take half a litre to top it up, adding RGJ will make it slightly less strong (both my wilco kits are 12%) but as it will make it taste better its not a bad trade off.
I am sure one of the more experienced members will be able to tell you how much lower the ABV will be if you do top up.
I am sure one of the more experienced members will be able to tell you how much lower the ABV will be if you do top up.