[otrs] question about using an external customer backend

Hello, I have read the documentation about using an external backend for customer data (http://doc.otrs.org/3.2/en/html/external-backends.html#customer-data ). My question now is, how can I map fields, that have different names? If for example the data field in OTRS is named email, but the corresponding value is stored in a field called emailaddress in our customer database, how can I map the two fields? Although I read the documentation more than once, it could find a solution for this. Thanks for any hints, Ralf