Yes, you can do it, my husband (the pig) does it all the time :-) One night he had 4 filets, gosh I was so embarassed Feel free to ask for what you want and if they can accomidate you, they certainly will. Just be sure to save some room for the yummy desserts, they are to die for :-)
As Donna said, don't worry. Usually in the dining room, no is not in the staff's vocabulary. My wife and I frequently travel with another couple. The husband is a bottomless pit (and skinny as a rail). I have seen him eat 3 appetizers, soup, salad, 3 lobster tails and 2 desserts. The only comment from the waiter was to ask him if he would like more.
Just one word of caution: if you don't exercise and you REALLY eat, you may be in for quite a shock when you return home and get on a scale. Enjoy!!!!
One thing we always try to do while cruising and eating way too much, is to use the stairs when ever possible instead of the elevators :-) Yes, you will be amazed at the scale when you get home, ugh!