As long as they are not severely malnourished, they should be perfectly healthy enough to through brumation safely. Turtles in the wild often survive the winter despite being almost emaciated. Based on the good care I notice you take with your Terrapene, I have a hard time believing they are underweight. It is better to err on the side of too little food than too much food before brumation, since they need to empty their gut completely before winter.