As of version 15 and 16:

• If the Employee does not have a related User record, they can only submit Expenses in the base currency of the Company.
  
• If the Employee does have a related User record, they can only submit Expenses in the base currency of any other Company they have access to.

In both cases, either the currency SYMBOL or currency ISO name (as show in your examples) are acceptable.

Note that any submitted expense can be edited once in Odoo, including changing the currency.

I have suggested via feedback that we instead let Employees email expenses in any active Currency in the Company, as this is probably what most Users expect.