Is it permissible to add a large sum to a small sum in a savings account to withdraw it all, and what about the accrued interest?

Savings accounts in usurious banks are considered unlawful usury (riba), which a Muslim must close and repent to Allah for. The interest gained from them must be disposed of by spending it on general Muslim welfare. It is not permissible to engage in such accounts for a small sum, as usury is one of the gravest sins.