DI: updating user-defined fields

I'm developing a java application to create & update sales orders in B1. I had planned on using user-defined fields to store cross-reference informatin when the sales order is created. This would allow me to process future updates. However, when I reviewed the DI API help, I discovered that user-defined fields are read-only. Is there a way to provide a value for a user-defined field when a business object (i.e. sales order) is created?

TIA

Russell