You can go to Tools > Options > System Options > Sketch and check the box at bottom for "Over defining dimensions > Set driven by default". That will keep you from getting that box every time. Then you should be able to insert extra dimensions without problems.
I'm not sure if you know this already but if you select "Make this dimension driven" in that box it will make it reference and not over-define the sketch. You can check the Don't ask again box or do what Glenn said to make it always do that.