Item 3 of:
states that an invoice reflect the price/value of an item in multiple currencies. The cardinality of Price is 0..1.
A user could put this into an extension on price, but we could add a new ASBIE:
AlternativeCurrencyPrice 0..n The price expressed in an alternative currency.
We cannot add AlternativeCurrencyLineExtensionAmount because we cannot use 0..n on a BBIE, so it would require introducing a new ABIE and ASBIE to it. But the cited reference appears to state that having just an alternative price is sufficient, so I think we don't also have to include the line extension amount in another currency.