It depends which 'package' deal you buy. Some packages have minimum ticket requirements. You can also always get a room only reservation, then buy tickets individually, and link them via
MDE. Often 'room' only discounts equal and package discounts offered. When looking on the Disney website, the site LIKES to add tickets any time you start looking at hotel rooms, but unless you are buying certain discounts or
DDP (not often a good deal any more)- you don't typically have to buy a package.
Still one more go around for your scenario if you want to book FP now for everyone is to buy three day tickets- then when you are at WDW, pay to add the no expire option to one ticket to keep the unsued days (is not a money saver, but look at what it costs to make it a ten day no expire maybe if that person will return to WDW).
Still one more option, is to wait until you are at WDW, then ADD the extra person to your room reservation. The last time I did this was under old FP, so I'm not positive of all the details now. You can't add DDP for that person, I'm not positive how it impacts your FP+ for that person. You CAN get Magic Express for that person by calling. But we were able to have that person listed on our room. Oh, they would also only get a grey Magicband, I think.
VERY IMPORTANT If you buy one day tickets then add days- you CAN'T add days AFTER the tickets are USED up. Your BEST bet is usually to upgrade them mid-morning or later ON THE DAY you use them from inside the park. (MK is best GS), but DON't FOGET!!! Typically, you won't be able to change them the day AFTER you use them or later.