Although you cannot currently link your AMEX account to Business Finance Assistant, you will be able to in the near future.
Access this link to check which other banks can be linked to Business Finance Assistant: https://token.io/coverage
Please note: you must first connect a Lloyds Bank Business Account to Business Finance Assistant before you can add bank accounts from other providers.
You can download your bank statements and import them to your Business Finance Assistant account by referring to this article: Importing transactions from bank statements or QIF, OFX or CSV files