Yes, you can trade in another currency without converting cash each time. To do this, change the Currency Handling settings for your desired currency from AutoFX to Manually.
When set to Manually, DEGIRO will not automatically convert funds to your base currency. For example, if Manually is enabled for USD, proceeds from the sale of a US security will remain in USD until converted manually by you.
Cash held in other currencies can be found in your Portfolio tab.
If you choose to invest again in USD, the amount will be taken from your USD money market fund.
If you wish to invest in another currency, you will need to convert the USD to that currency, or back to your base currency if AutoFX is enabled for your desired currency. To withdraw your funds, you will first need to convert your USD back to your base currency.