BERTH, BEGAN, KNEEL, SUITE, SQUAD, CHEWY, CELLO, PILOT, WIELD. The added letters spell TELESCOPE.
HERB + T becomes BERTH
BANG + E becomes BEGAN
KEEN + L becomes KNEEL
SUIT + E becomes SUITE
QUAD + S becomes SQUAD
WHEY + C becomes CHEWY
CELL + O becomes CELLO
TOIL + P becomes PILOT
WILD + E becomes WIELD