I don't think you can.
In your case, Jim, you are taking a group and therefore the cost of the tips has probably been added to the price of the cruise.....lot of paperwork for a TA.
Mikey - if you want to pre-pay your tips, the only way I know to do it, is to go the Front desk when you board the ship and give them either cash or travellers cheques in the amount that your tips will work out to. That is $10.00 per day per person. so if there are 2 of you on a 7 day cruise, you can deposit $140.00. 10 day cruise would be $200.00
I don't know what kind of a system RCI uses. The above is the only way I have heard of "pre-paying" your tips.