I’m guessing you want to use the (previously HFS+ formatted) disks in your array, while keeping the data? That’s not possible, no. BTRFS can convert an existing EXT filesystem to BTRFS, but I’m not even sure if that works for a single disk added to an existing array, but eitherway: HFS+ is right out. You’d really have to store the data elsewhere, add the drive and have it formatted and made part of the pool, rebalanced etc, and then you could put the data back by storing it in a share in your pool. Or, if your pool is currently big enough, store the data on it first and then add the drive.