You have a few options on how to add money to your USD account:
SWIFT transfer from another USD account. Incoming SWIFT transfers cost $6.24.
ACH transfers with your Routing and Account Number.
Transfer from your GBP nsave account on the app via "exchange". Any currency exchange (FX) fees are live charges and will be shown on the app before you complete the transaction.
Transfer from your USD investment account into your main USD nsave account.