Yes. You can cover some of a Flex expense with your credit line and some with an ACH payment. Simply log in to your dashboard in the grace period, choose the Flex Pay expense to repay, and set the amount to repay from your bank account or credit card.
At the end of the grace period, the rest will continue as a standard draw according to the default repayment terms, up to your available credit limit.
Remember, you can change this default anytime and the updates selection will apply to future—but not past—Flex expenses.