Association/navigation for non-key fields of entities

Dear Experts,

I am building a service for Sales order. My structure as follows -

Entities:

     1. VBAK

               Key :           vbeln

               non-key :      kunnr

               -- there is no matnr

     2. VBAP

               Key :           vbeln

               non-key:      matnr

               -- there is no kunnr

     3. KNA1

                Key :           kunnr

     4. MARA

                Key :           matnr

We have association exist already for VBAP and VBAK.

now my requirement is to get VBAK and its corresponding KNA1. Similar way VBAP --> MARA

How can we do the association/navigation using non-key fields?

Thank you,

Murali