The best way would be to create a <xs:complexType> in your schema.
I am not sure how to do this with the Biztalk Xml editor, I tend to use a text editor.
Open your schema in a text editor and find you definition for Currency, it should look something like this:
<xs:element name="Currency">
<xs:complexType>
…
</xs:complexType>
</xs:element>
Cut and paste the complexType as a child of <xs:schema> and give it a name
<xs:schema>
…
<xs:complexType name="CurrencyType">
…
</xs:complexType>
<xs:schema>
Modify the original Currency element to use this type:
<xs:element name="Currency" type="CurrencyType">
</xs:element>
When you open the schema in the Biztalk Xml Editor the Currency element should have a data type of CurrencyType and you should be able to set any other element to this same type.