PF doesn't allow for replacements. Typically what we do is have people agree to a full run up front when they join of and ask if they have to leave for anything within that allotted period. If they tell us upfront that they can only stay a bit (at least half the timer) then we bring them along. If they just up and quit in the middle of the duty (voluntarily speaking) then we usually block them from joining future pfs and wasting people's time.

But in regards to the question you can't refresh in PF at all to my understating in any duty.