There's an option found under the 'Integration' tab of your 'Account Settings' menu. It is called 'Publish multiple currencies'.
If you enable this then your items will not be converted by Receipt Bank into your 'Default currency' on publishing to Xero.
These items will remain in their original currency when published and then will show up in Xero in their original currency, provided that your Xero is also a multi currency account.
If your Xero account doesn't have multi currency functionality then even with 'Publish multiple currencies' enabled these items will be converted to your base currency by Receipt Bank on publishing them.