I have a Belgian beer that started with regular Belgian yeast then I transferred to a bucket and added Wyeast 5526 Brettanomyces lambicus. It has aged for nine months. I want to re-ferment in a a bottle using fresh wort from a current batch. Should I pitch fresh neutral yeast (safale us-05 for instance) into the fresh wort and combine at high Krausen mix with the aged beer and bottle or simply add the fresh wort and bottle and let the bret take care of the carbonation.
It will almost certainly carbonate on its own, but neutral ale yeast will speed things up.
I’d suggest just using regular priming or table sugar for carbonation. It is hard to predict how much of the “unfermentables” in the wort the Brett will consume in the long term, leading to over-carbonation. Otherwise you’ll have to aim low, and it could be a couple months even with the ale yeast until the beer reaches full carbonation.