knieves - Welcome Aboard to Cruise-Chat - A place where I hope you will find the time to spend many happy hours ashore and a few at sea!! This is the place to be when you are "on the beach" and " 'tween ships"!
In certain past cruises I have added an additional passenger to my stateroom after the original booking. So long as the stateroom has the capacity for an additional passenger the fare is adjusted accordingly to the cruise fare for that cabin with the new numbers of passenger occupancy. If you are a solo cruiser already paying 200% - the additional cost will likely be the added on taxes and port fees. If you are adding a third/fourth passenger then there will be additional fares to be paid, usually at a different rate!
In all cases you must be making the "add on" reservation within the time limits established by the cruiseline and Homeland Security - you can't walk up to the purser the day of the sailing and add another passenger to the ship's manifest - It does not work like an airline!!!
I recommend you contact your TA or the cruiseline for making the arrangements! Best of Luck to you, Welcome Aboard and...