To do this, enable manual currency handling in your Currency Handling settings. Once you have set a currency to Manually, DEGIRO will not perform automatic conversions between this currency and the base currency of your account.
Funds held in other currencies will be shown in your Portfolio tab.
Please note that deposits and withdrawals will be made in the base currency of your account. This means that in order to withdraw funds, you will first need to convert them back to your base currency.