You can fund your account in a currency other than the account currency. Deposits in any currency which differs from your base account currency will be subjected to a conversion fee.
The conversion fee is calculated by using the mid-spread FX spot rate* at the time of transfer, plus/minus the rates that you can find in the Currency Conversion Fees Section.
*Spot rate refers to the inter-bank rate at the point of execution.
If you only have a EUR account but wish to fund it in USD and retain the USD without conversion, you can open a USD sub-account. This will enable you to send USD directly to the sub-account.