If there are only a few, and they don’t have reams of replies, you could consider moving all the replies to a new topic (using the topic wrench ‘select posts’), adding a new one below the OP, and then moving them back (plus a little clean-up of the small post actions). It would send out some notifications though, so may be quite ‘noisy’.
Though I feel like I’m missing something obvious. 
Can anyone else think of a more elegant solution?