Currency selection in custom fields

I would like to add a currency selection to custom fields so that the user can choose their default currency on registration. How would I make the field update the users default currency. If I try pointing the field to the user_currency_id column in the user table I get The field "user_currency_id" already exists in the table "user"?

There is no user default currency in HikaShop.

You would need to create a joomla user plugin to override the default currency based on the user's currency when the user logs on the frontend.
Then you would simply need a user custom field to let the user select his currency during the registration and use it in your custom plugin.