Create a custom currency via SDK

Hi Everyone

I am creating a web service to create/update records in the transactioncurrency entity via SDK, with other entities this work fine, but when I try with this entity calling the Service.Create() or Service.Update throw me an error "unexpected error".

For the especific requeriments we need create records with customs codes, for example, if exists the USD currency for Spain, I have to generate a code like this UES, first char from currency code and the ISO code for the country, If I would made from the standard form I can do it  selecting the "custom currency" radio button, I think this is the reason why I can't do it via SDK but I have not find the way to set this type of currency.

if somebody know how do it this I will be gratefull

Thanks in advance

Jose Fuentes