You can definitely order extra entrees, however not sure about modifying them.
I was having a hard time deciding between the regular eggs benedict and the lobster eggs benedict in March. I asked my server if I could just have half an order of the regular benedict and half of the lobster and was told I couldn't, but I could just order two entrees.
I knew there was no way on a good day I'd be able to eat both entrees - let alone when there was the amazing buffet as well, so just went with the lobster - I could not be that wasteful even if they did seem to encourage it.
PS - The brunch at Cali Grill is AMAZING and I'm so glad I did it.
We're trying the one at Narcoossee's next week (where you only get one entrée and no buffet). I hope it lives up to the brunch at Cali Grill.