Don't plan on being able to do that, unless you are booking "low-demand" FP attractions.
While it is allowed to try and "copy" FP+ reservations over to additional guests, if those spots at those times are already booked and full, the copy won't be allowed.
A workaround of sorts is to try to book FP+ for the additional guest(s) which times OVERLAP with your current FP+ times so that you can all enter during that overlapping time window.
But again, overall, don't expect to be able to make these work for attractions which historically "run out" of FP+ in advance.
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