Splitting Viseme, or adding new viseme

If you hold down the Option (Mac) or Alt (Win) key while the cursor is above the viseme bar, it'll change to a razor cursor to let you split a viseme in two at a specific time, then you can just right click above the viseme to replace as usual. You can do the split & replace with a different viseme in one step by right-clicking while holding down Option/Alt. Hope this helps.