Hi,
in SPRO go to CABS-> Extensions and Field Control (Personalization) -> Configure Generic Field Display on User Interface.
Enter your custom field in the table and tick the roundtrip checkbox.
If you want to do it by yourself take a look at the mapper class with the following methods:
REFRESH = update the Webdynpro with the object values
UPDATE = update the object with the Webdynpro values
Regards,
Robin