Adding extra grains will increase your gravity, but not your efficiency.
Are you using some form of brewing software? If so, you should be able to set the efficiency to what you get, and the software will predict the OG for a given volume and grain bill.
If you want to increase your OG by 10%, then increasing the base malt by 10% should get you pretty close (assuming you are doing all grain).
There are only 10 types of people in this world. Those that understand binary, and those that don't.