I think you're probably going to have to empty all bottles and RE-bottle.
That's an option but there would be too much risk of oxidation and a lot of work to pour all the bottles back into the bottling bucket.
I had a similar problem when I started brewing, I under calculated the priming sugar and had very little gas after two weeks.
I think I forgot to tare the scales when I was weighing the sugar.
Anyway I made a guess of adding something like an extra 1.5 grams per bottle and just dissolved it in boiling water and added a few ml to each bottle with a pipette. Luckily the beer ending up perfectly carbed after about 10 more days.
Using the same method for adding extra yeast would be less complicated because you just need to make sure you have enough liquid to add a few ml to each bottle.
I don't know if you have any original yeast left if you have then just make a small starter and add a few ml to each bottle.
Or else get a pack of dry yeast with high alc tolerance and a lower attenuation than the original yeast, S-33 or T-58 for example, and rehydrate in a few hundred ml, give it a bit of a stir to make sure it's evenly distributed and then add a few ml to each bottle.
I have a bottle opener that barely damages the cap so I could reuse the same cap. Just make sure you properly sanitise everything and do one bottle at a time (open, add yeast, close).
Whatever you decide I hope it works out.
Any suggestion ?
